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

deps/deps.go

@@ -1,17 +1,18 @@
 package deps
 
 import (
-	"io/ioutil"
-	"log"
-	"os"
 	"time"
 
+	"github.com/gohugoio/hugo/common/loggers"
+
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/langs"
+	"github.com/gohugoio/hugo/media"
 	"github.com/gohugoio/hugo/metrics"
 	"github.com/gohugoio/hugo/output"
+	"github.com/gohugoio/hugo/resource"
 	"github.com/gohugoio/hugo/source"
 	"github.com/gohugoio/hugo/tpl"
 	jww "github.com/spf13/jwalterweatherman"
@@ -30,6 +31,9 @@ type Deps struct {
 	// The templates to use. This will usually implement the full tpl.TemplateHandler.
 	Tmpl tpl.TemplateFinder `json:"-"`
 
+	// We use this to parse and execute ad-hoc text templates.
+	TextTmpl tpl.TemplateParseFinder `json:"-"`
+
 	// The file systems to use.
 	Fs *hugofs.Fs `json:"-"`
 
@@ -42,6 +46,9 @@ type Deps struct {
 	// The SourceSpec to use
 	SourceSpec *source.SourceSpec `json:"-"`
 
+	// The Resource Spec to use
+	ResourceSpec *resource.Spec
+
 	// The configuration to use
 	Cfg config.Provider `json:"-"`
 
@@ -115,7 +122,7 @@ func New(cfg DepsCfg) (*Deps, error) {
 	}
 
 	if logger == nil {
-		logger = jww.NewNotepad(jww.LevelError, jww.LevelError, os.Stdout, ioutil.Discard, "", log.Ldate|log.Ltime)
+		logger = loggers.NewErrorLogger()
 	}
 
 	if fs == nil {
@@ -129,6 +136,11 @@ func New(cfg DepsCfg) (*Deps, error) {
 		return nil, err
 	}
 
+	resourceSpec, err := resource.NewSpec(ps, logger, cfg.MediaTypes)
+	if err != nil {
+		return nil, err
+	}
+
 	contentSpec, err := helpers.NewContentSpec(cfg.Language)
 	if err != nil {
 		return nil, err
@@ -153,6 +165,7 @@ func New(cfg DepsCfg) (*Deps, error) {
 		PathSpec:            ps,
 		ContentSpec:         contentSpec,
 		SourceSpec:          sp,
+		ResourceSpec:        resourceSpec,
 		Cfg:                 cfg.Language,
 		Language:            cfg.Language,
 		Timeout:             time.Duration(timeoutms) * time.Millisecond,
@@ -167,7 +180,8 @@ func New(cfg DepsCfg) (*Deps, error) {
 
 // ForLanguage creates a copy of the Deps with the language dependent
 // parts switched out.
-func (d Deps) ForLanguage(l *langs.Language) (*Deps, error) {
+func (d Deps) ForLanguage(cfg DepsCfg) (*Deps, error) {
+	l := cfg.Language
 	var err error
 
 	d.PathSpec, err = helpers.NewPathSpecWithBaseBaseFsProvided(d.Fs, l, d.BaseFs)
@@ -180,6 +194,11 @@ func (d Deps) ForLanguage(l *langs.Language) (*Deps, error) {
 		return nil, err
 	}
 
+	d.ResourceSpec, err = resource.NewSpec(d.PathSpec, d.Log, cfg.MediaTypes)
+	if err != nil {
+		return nil, err
+	}
+
 	d.Cfg = l
 	d.Language = l
 
@@ -212,6 +231,9 @@ type DepsCfg struct {
 	// The configuration to use.
 	Cfg config.Provider
 
+	// The media types configured.
+	MediaTypes media.Types
+
 	// Template handling.
 	TemplateProvider ResourceProvider
 	WithTemplate     func(templ tpl.TemplateHandler) error