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

helpers/general.go

@@ -356,7 +356,7 @@ func MD5String(f string) string {
 // MD5FromFileFast creates a MD5 hash from the given file. It only reads parts of
 // the file for speed, so don't use it if the files are very subtly different.
 // It will not close the file.
-func MD5FromFileFast(f afero.File) (string, error) {
+func MD5FromFileFast(r io.ReadSeeker) (string, error) {
 	const (
 		// Do not change once set in stone!
 		maxChunks = 8
@@ -369,7 +369,7 @@ func MD5FromFileFast(f afero.File) (string, error) {
 
 	for i := 0; i < maxChunks; i++ {
 		if i > 0 {
-			_, err := f.Seek(seek, 0)
+			_, err := r.Seek(seek, 0)
 			if err != nil {
 				if err == io.EOF {
 					break
@@ -378,7 +378,7 @@ func MD5FromFileFast(f afero.File) (string, error) {
 			}
 		}
 
-		_, err := io.ReadAtLeast(f, buff, peekSize)
+		_, err := io.ReadAtLeast(r, buff, peekSize)
 		if err != nil {
 			if err == io.EOF || err == io.ErrUnexpectedEOF {
 				h.Write(buff)

helpers/path.go

@@ -90,6 +90,11 @@ func (p *PathSpec) MakePathSanitized(s string) string {
 	return strings.ToLower(p.MakePath(s))
 }
 
+// ToSlashTrimLeading is just a filepath.ToSlaas with an added / prefix trimmer.
+func ToSlashTrimLeading(s string) string {
+	return strings.TrimPrefix(filepath.ToSlash(s), "/")
+}
+
 // MakeTitle converts the path given to a suitable title, trimming whitespace
 // and replacing hyphens with whitespace.
 func MakeTitle(inpath string) string {
@@ -222,12 +227,22 @@ func GetDottedRelativePath(inPath string) string {
 	return dottedPath
 }
 
+// ExtNoDelimiter takes a path and returns the extension, excluding the delmiter, i.e. "md".
+func ExtNoDelimiter(in string) string {
+	return strings.TrimPrefix(Ext(in), ".")
+}
+
 // Ext takes a path and returns the extension, including the delmiter, i.e. ".md".
 func Ext(in string) string {
 	_, ext := fileAndExt(in, fpb)
 	return ext
 }
 
+// PathAndExt is the same as FileAndExt, but it uses the path package.
+func PathAndExt(in string) (string, string) {
+	return fileAndExt(in, pb)
+}
+
 // FileAndExt takes a path and returns the file and extension separated,
 // the extension including the delmiter, i.e. ".md".
 func FileAndExt(in string) (string, string) {

helpers/path_test.go

@@ -78,6 +78,9 @@ func TestMakePathSanitized(t *testing.T) {
 	v.Set("dataDir", "data")
 	v.Set("i18nDir", "i18n")
 	v.Set("layoutDir", "layouts")
+	v.Set("assetDir", "assets")
+	v.Set("resourceDir", "resources")
+	v.Set("publishDir", "public")
 	v.Set("archetypeDir", "archetypes")
 
 	l := langs.NewDefaultLanguage(v)
@@ -475,6 +478,7 @@ func createTempDirWithNonZeroLengthFiles() (string, error) {
 		return "", fileErr
 	}
 	byteString := []byte("byteString")
+
 	fileErr = ioutil.WriteFile(f.Name(), byteString, 0644)
 	if fileErr != nil {
 		// delete the file
@@ -585,6 +589,11 @@ func TestAbsPathify(t *testing.T) {
 
 }
 
+func TestExtNoDelimiter(t *testing.T) {
+	assert := require.New(t)
+	assert.Equal("json", ExtNoDelimiter(filepath.FromSlash("/my/data.json")))
+}
+
 func TestFilename(t *testing.T) {
 	type test struct {
 		input, expected string

helpers/testhelpers_test.go

@@ -38,6 +38,9 @@ func newTestCfg() *viper.Viper {
 	v.Set("dataDir", "data")
 	v.Set("i18nDir", "i18n")
 	v.Set("layoutDir", "layouts")
+	v.Set("assetDir", "assets")
+	v.Set("resourceDir", "resources")
+	v.Set("publishDir", "public")
 	v.Set("archetypeDir", "archetypes")
 	return v
 }