hugolib: Extract date and slug from filename

This commit makes it possible to extract the date from the content filename. Also, the filenames in these cases will make for very poor permalinks, so we will also use the remaining part as the page `slug` if that value is not set in front matter.

This should make it easier to move content from Jekyll to Hugo.

To enable, put this in your `config.toml`:

```toml
[frontmatter]
date  = [":filename", ":default"]
```

This commit is also a spring cleaning of how the different dates are configured in Hugo. Hugo will check for dates following the configuration from left to right, starting with `:filename` etc.

So, if you want to use the `file modification time`, this can be a good configuration:

 ```toml
[frontmatter]
date = [ "date",":fileModTime", ":default"]
lastmod = ["lastmod" ,":fileModTime", ":default"]
```

The current `:default` values for the different dates are

```toml
[frontmatter]
date = ["date","publishDate", "lastmod"]
lastmod = ["lastmod", "date","publishDate"]
publishDate = ["publishDate", "date"]
expiryDate = ["expiryDate"]
```

The above will now be the same as:

```toml
[frontmatter]
date = [":default"]
lastmod = [":default"]
publishDate = [":default"]
expiryDate = [":default"]
```

Note:

* We have some built-in aliases to the above: lastmod => modified, publishDate => pubdate, published and expiryDate => unpublishdate.
* If you want a new configuration for, say, `date`, you can provide only that line, and the rest will be preserved.
* All the keywords to the right that does not start with a ":" maps to front matter parameters, and can be any date param (e.g. `myCustomDateParam`).
* The keywords to the left are the **4 predefined dates in Hugo**, i.e. they are constant values.
* The current "special date handlers" are `:fileModTime` and `:filename`. We will soon add `:git` to that list.

Fixes #285
Closes #3310
Closes #3762
Closes #4340

hugolib/page.go

@@ -1,4 +1,4 @@
-// Copyright 2016 The Hugo Authors. All rights reserved.
+// Copyright 2018 The Hugo Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@ import (
 	"github.com/bep/gitmap"
 
 	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/hugolib/pagemeta"
 	"github.com/gohugoio/hugo/resource"
 
 	"github.com/gohugoio/hugo/output"
@@ -140,9 +141,6 @@ type Page struct {
 	Draft     bool
 	Status    string
 
-	PublishDate time.Time
-	ExpiryDate  time.Time
-
 	// PageMeta contains page stats such as word count etc.
 	PageMeta
 
@@ -223,11 +221,12 @@ type Page struct {
 	Keywords    []string
 	Data        map[string]interface{}
 
-	Date    time.Time
-	Lastmod time.Time
+	pagemeta.PageDates
 
 	Sitemap Sitemap
-	URLPath
+	pagemeta.URLPath
+	frontMatterURL string
+
 	permalink    string
 	relPermalink string
 
@@ -1115,12 +1114,44 @@ func (p *Page) update(frontmatter map[string]interface{}) error {
 	// Needed for case insensitive fetching of params values
 	helpers.ToLowerMap(frontmatter)
 
-	var modified time.Time
+	var mtime time.Time
+	if p.Source.FileInfo() != nil {
+		mtime = p.Source.FileInfo().ModTime()
+	}
+
+	descriptor := &pagemeta.FrontMatterDescriptor{
+		Frontmatter:  frontmatter,
+		Params:       p.params,
+		Dates:        &p.PageDates,
+		PageURLs:     &p.URLPath,
+		BaseFilename: p.BaseFileName(),
+		ModTime:      mtime}
+
+	// Handle the date separately
+	// TODO(bep) we need to "do more" in this area so this can be split up and
+	// more easily tested without the Page, but the coupling is strong.
+	err := p.s.frontmatterHandler.HandleDates(descriptor)
+	if err != nil {
+		p.s.Log.ERROR.Printf("Failed to handle dates for page %q: %s", p.Path(), err)
+	}
 
-	var err error
 	var draft, published, isCJKLanguage *bool
 	for k, v := range frontmatter {
 		loki := strings.ToLower(k)
+
+		if loki == "published" { // Intentionally undocumented
+			vv, err := cast.ToBoolE(v)
+			if err == nil {
+				published = &vv
+			}
+			// published may also be a date
+			continue
+		}
+
+		if p.s.frontmatterHandler.IsDateKey(loki) {
+			continue
+		}
+
 		switch loki {
 		case "title":
 			p.title = cast.ToString(v)
@@ -1139,7 +1170,7 @@ func (p *Page) update(frontmatter map[string]interface{}) error {
 				return fmt.Errorf("Only relative URLs are supported, %v provided", url)
 			}
 			p.URLPath.URL = cast.ToString(v)
-			p.URLPath.frontMatterURL = p.URLPath.URL
+			p.frontMatterURL = p.URLPath.URL
 			p.params[loki] = p.URLPath.URL
 		case "type":
 			p.contentType = cast.ToString(v)
@@ -1150,12 +1181,6 @@ func (p *Page) update(frontmatter map[string]interface{}) error {
 		case "keywords":
 			p.Keywords = cast.ToStringSlice(v)
 			p.params[loki] = p.Keywords
-		case "date":
-			p.Date, err = cast.ToTimeE(v)
-			if err != nil {
-				p.s.Log.ERROR.Printf("Failed to parse date '%v' in page %s", v, p.File.Path())
-			}
-			p.params[loki] = p.Date
 		case "headless":
 			// For now, only the leaf bundles ("index.md") can be headless (i.e. produce no output).
 			// We may expand on this in the future, but that gets more complex pretty fast.
@@ -1163,19 +1188,6 @@ func (p *Page) update(frontmatter map[string]interface{}) error {
 				p.headless = cast.ToBool(v)
 			}
 			p.params[loki] = p.headless
-		case "lastmod":
-			p.Lastmod, err = cast.ToTimeE(v)
-			if err != nil {
-				p.s.Log.ERROR.Printf("Failed to parse lastmod '%v' in page %s", v, p.File.Path())
-			}
-		case "modified":
-			vv, err := cast.ToTimeE(v)
-			if err == nil {
-				p.params[loki] = vv
-				modified = vv
-			} else {
-				p.params[loki] = cast.ToString(v)
-			}
 		case "outputs":
 			o := cast.ToStringSlice(v)
 			if len(o) > 0 {
@@ -1190,34 +1202,9 @@ func (p *Page) update(frontmatter map[string]interface{}) error {
 				}
 
 			}
-		case "publishdate", "pubdate":
-			p.PublishDate, err = cast.ToTimeE(v)
-			if err != nil {
-				p.s.Log.ERROR.Printf("Failed to parse publishdate '%v' in page %s", v, p.File.Path())
-			}
-			p.params[loki] = p.PublishDate
-		case "expirydate", "unpublishdate":
-			p.ExpiryDate, err = cast.ToTimeE(v)
-			if err != nil {
-				p.s.Log.ERROR.Printf("Failed to parse expirydate '%v' in page %s", v, p.File.Path())
-			}
 		case "draft":
 			draft = new(bool)
 			*draft = cast.ToBool(v)
-		case "published": // Intentionally undocumented
-			vv, err := cast.ToBoolE(v)
-			if err == nil {
-				published = &vv
-			} else {
-				// Some sites use this as the publishdate
-				vv, err := cast.ToTimeE(v)
-				if err == nil {
-					p.PublishDate = vv
-					p.params[loki] = p.PublishDate
-				} else {
-					p.params[loki] = cast.ToString(v)
-				}
-			}
 		case "layout":
 			p.Layout = cast.ToString(v)
 			p.params[loki] = p.Layout
@@ -1333,32 +1320,6 @@ func (p *Page) update(frontmatter map[string]interface{}) error {
 	}
 	p.params["draft"] = p.Draft
 
-	if p.Date.IsZero() {
-		p.Date = p.PublishDate
-	}
-
-	if p.PublishDate.IsZero() {
-		p.PublishDate = p.Date
-	}
-
-	if p.Date.IsZero() && p.s.Cfg.GetBool("useModTimeAsFallback") {
-		p.Date = p.Source.FileInfo().ModTime()
-	}
-
-	if p.Lastmod.IsZero() {
-		if !modified.IsZero() {
-			p.Lastmod = modified
-		} else {
-			p.Lastmod = p.Date
-		}
-
-	}
-
-	p.params["date"] = p.Date
-	p.params["lastmod"] = p.Lastmod
-	p.params["publishdate"] = p.PublishDate
-	p.params["expirydate"] = p.ExpiryDate
-
 	if isCJKLanguage != nil {
 		p.isCJKLanguage = *isCJKLanguage
 	} else if p.s.Cfg.GetBool("hasCJKLanguage") {
@@ -1865,14 +1826,6 @@ func (p *Page) String() string {
 	return fmt.Sprintf("Page(%q)", p.title)
 }
 
-type URLPath struct {
-	URL            string
-	frontMatterURL string
-	Permalink      string
-	Slug           string
-	Section        string
-}
-
 // Scratch returns the writable context associated with this Page.
 func (p *Page) Scratch() *Scratch {
 	if p.scratch == nil {