Add Hugo Piper with SCSS support and much more

Before this commit, you would have to use page bundles to do image processing etc. in Hugo.

This commit adds

* A new `/assets` top-level project or theme dir (configurable via `assetDir`)
* A new template func, `resources.Get` which can be used to "get a resource" that can be further processed.

This means that you can now do this in your templates (or shortcodes):

```bash
{{ $sunset := (resources.Get "images/sunset.jpg").Fill "300x200" }}
```

This also adds a new `extended` build tag that enables powerful SCSS/SASS support with source maps. To compile this from source, you will also need a C compiler installed:

```
HUGO_BUILD_TAGS=extended mage install
```

Note that you can use output of the SCSS processing later in a non-SCSSS-enabled Hugo.

The `SCSS` processor is a _Resource transformation step_ and it can be chained with the many others in a pipeline:

```bash
{{ $css := resources.Get "styles.scss" | resources.ToCSS | resources.PostCSS | resources.Minify | resources.Fingerprint }}
<link rel="stylesheet" href="{{ $styles.RelPermalink }}" integrity="{{ $styles.Data.Digest }}" media="screen">
```

The transformation funcs above have aliases, so it can be shortened to:

```bash
{{ $css := resources.Get "styles.scss" | toCSS | postCSS | minify | fingerprint }}
<link rel="stylesheet" href="{{ $styles.RelPermalink }}" integrity="{{ $styles.Data.Digest }}" media="screen">
```

A quick tip would be to avoid the fingerprinting part, and possibly also the not-superfast `postCSS` when you're doing development, as it allows Hugo to be smarter about the rebuilding.

Documentation will follow, but have a look at the demo repo in https://github.com/bep/hugo-sass-test

New functions to create `Resource` objects:

* `resources.Get` (see above)
* `resources.FromString`: Create a Resource from a string.

New `Resource` transformation funcs:

* `resources.ToCSS`: Compile `SCSS` or `SASS` into `CSS`.
* `resources.PostCSS`: Process your CSS with PostCSS. Config file support (project or theme or passed as an option).
* `resources.Minify`: Currently supports `css`, `js`, `json`, `html`, `svg`, `xml`.
* `resources.Fingerprint`: Creates a fingerprinted version of the given Resource with Subresource Integrity..
* `resources.Concat`: Concatenates a list of Resource objects. Think of this as a poor man's bundler.
* `resources.ExecuteAsTemplate`: Parses and executes the given Resource and data context (e.g. .Site) as a Go template.

Fixes #4381
Fixes #4903
Fixes #4858

media/mediaType.go

@@ -50,7 +50,8 @@ func FromString(t string) (Type, error) {
 	mainType := parts[0]
 	subParts := strings.Split(parts[1], "+")
 
-	subType := subParts[0]
+	subType := strings.Split(subParts[0], ";")[0]
+
 	var suffix string
 
 	if len(subParts) == 1 {
@@ -85,25 +86,38 @@ func (m Type) FullSuffix() string {
 var (
 	CalendarType   = Type{"text", "calendar", "ics", defaultDelimiter}
 	CSSType        = Type{"text", "css", "css", defaultDelimiter}
+	SCSSType       = Type{"text", "x-scss", "scss", defaultDelimiter}
+	SASSType       = Type{"text", "x-sass", "sass", defaultDelimiter}
 	CSVType        = Type{"text", "csv", "csv", defaultDelimiter}
 	HTMLType       = Type{"text", "html", "html", defaultDelimiter}
 	JavascriptType = Type{"application", "javascript", "js", defaultDelimiter}
 	JSONType       = Type{"application", "json", "json", defaultDelimiter}
 	RSSType        = Type{"application", "rss", "xml", defaultDelimiter}
 	XMLType        = Type{"application", "xml", "xml", defaultDelimiter}
-	TextType       = Type{"text", "plain", "txt", defaultDelimiter}
+	// The official MIME type of SVG is image/svg+xml. We currently only support one extension
+	// per mime type. The workaround in projects is to create multiple media type definitions,
+	// but we need to improve this to take other known suffixes into account.
+	// But until then, svg has an svg extension, which is very common. TODO(bep)
+	SVGType  = Type{"image", "svg", "svg", defaultDelimiter}
+	TextType = Type{"text", "plain", "txt", defaultDelimiter}
+
+	OctetType = Type{"application", "octet-stream", "", ""}
 )
 
 var DefaultTypes = Types{
 	CalendarType,
 	CSSType,
 	CSVType,
+	SCSSType,
+	SASSType,
 	HTMLType,
 	JavascriptType,
 	JSONType,
 	RSSType,
 	XMLType,
+	SVGType,
 	TextType,
+	OctetType,
 }
 
 func init() {
@@ -125,6 +139,16 @@ func (t Types) GetByType(tp string) (Type, bool) {
 	return Type{}, false
 }
 
+// GetFirstBySuffix will return the first media type matching the given suffix.
+func (t Types) GetFirstBySuffix(suffix string) (Type, bool) {
+	for _, tt := range t {
+		if strings.EqualFold(suffix, tt.Suffix) {
+			return tt, true
+		}
+	}
+	return Type{}, false
+}
+
 // GetBySuffix gets a media type given as suffix, e.g. "html".
 // It will return false if no format could be found, or if the suffix given
 // is ambiguous.