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

hugofs/hashing_fs.go

@@ -0,0 +1,96 @@
+// 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.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugofs
+
+import (
+	"crypto/md5"
+	"encoding/hex"
+	"hash"
+	"os"
+
+	"github.com/spf13/afero"
+)
+
+var (
+	_ afero.Fs = (*md5HashingFs)(nil)
+)
+
+// FileHashReceiver will receive the filename an the content's MD5 sum on file close.
+type FileHashReceiver interface {
+	OnFileClose(name, md5sum string)
+}
+
+type md5HashingFs struct {
+	afero.Fs
+	hashReceiver FileHashReceiver
+}
+
+// NewHashingFs creates a new filesystem that will receive MD5 checksums of
+// any written file content on Close. Note that this is probably not a good
+// idea for "full build" situations, but when doing fast render mode, the amount
+// of files published is low, and it would be really nice to know exactly which
+// of these files where actually changed.
+// Note that this will only work for file operations that use the io.Writer
+// to write content to file, but that is fine for the "publish content" use case.
+func NewHashingFs(delegate afero.Fs, hashReceiver FileHashReceiver) afero.Fs {
+	return &md5HashingFs{Fs: delegate, hashReceiver: hashReceiver}
+}
+
+func (fs *md5HashingFs) Create(name string) (afero.File, error) {
+	f, err := fs.Fs.Create(name)
+	if err == nil {
+		f = fs.wrapFile(f)
+	}
+	return f, err
+}
+
+func (fs *md5HashingFs) OpenFile(name string, flag int, perm os.FileMode) (afero.File, error) {
+	f, err := fs.Fs.OpenFile(name, flag, perm)
+	if err == nil && isWrite(flag) {
+		f = fs.wrapFile(f)
+	}
+	return f, err
+}
+
+func (fs *md5HashingFs) wrapFile(f afero.File) afero.File {
+	return &hashingFile{File: f, h: md5.New(), hashReceiver: fs.hashReceiver}
+}
+
+func isWrite(flag int) bool {
+	return flag&os.O_RDWR != 0 || flag&os.O_WRONLY != 0
+}
+
+func (fs *md5HashingFs) Name() string {
+	return "md5HashingFs"
+}
+
+type hashingFile struct {
+	hashReceiver FileHashReceiver
+	h            hash.Hash
+	afero.File
+}
+
+func (h *hashingFile) Write(p []byte) (n int, err error) {
+	n, err = h.File.Write(p)
+	if err != nil {
+		return
+	}
+	return h.h.Write(p)
+}
+
+func (h *hashingFile) Close() error {
+	sum := hex.EncodeToString(h.h.Sum(nil))
+	h.hashReceiver.OnFileClose(h.Name(), sum)
+	return h.File.Close()
+}