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

90 files changed, 4686 insertions, 1126 deletions

diff --git a/.gitignore b/.gitignore
index 08e830c87..032a643c9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,5 +15,7 @@ vendor/*/
 *.debug
 coverage*.out
 
+dock.sh
+
 GoBuilds
 dist
diff --git a/.travis.yml b/.travis.yml
index b04528203..f398f2015 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,8 @@
 language: go
 sudo: false
 dist: trusty
+env:
+  HUGO_BUILD_TAGS="extended"
 git:
   depth: false
 go:
@@ -18,8 +20,9 @@ install:
   - go get github.com/magefile/mage
   - mage -v vendor
 script:
-  - mage -v hugoRace
+  - mage -v test
   - mage -v check
+  - mage -v hugo
   - ./hugo -s docs/
   - ./hugo --renderToMemory -s docs/
 before_install:
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ad33414b5..87199a411 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -192,6 +192,12 @@ To list all available commands along with descriptions:
 mage -l
 ```
 
+**Note:** From Hugo 0.43 we have added a build tag, `extended` that adds **SCSS support**. This needs a C compiler installed to build. You can enable this when building by:
+
+```bash
+HUGO_BUILD_TAGS=extended mage install
+````
+
 ### Updating the Hugo Sources
 
 If you want to stay in sync with the Hugo repository, you can easily pull down
diff --git a/Dockerfile b/Dockerfile
index 919b02237..919b02237 100644..100755
--- a/Dockerfile
+++ b/Dockerfile
diff --git a/Gopkg.lock b/Gopkg.lock
index 51fb96c52..8d07f2390 100644
--- a/Gopkg.lock
+++ b/Gopkg.lock
@@ -3,6 +3,12 @@
 
 [[projects]]
   branch = "master"
+  name = "github.com/BurntSushi/locker"
+  packages = ["."]
+  revision = "a6e239ea1c69bff1cfdb20c4b73dadf52f784b6a"
+
+[[projects]]
+  branch = "master"
   name = "github.com/BurntSushi/toml"
   packages = ["."]
   revision = "a368813c5e648fee92e5f6c30e3944ff9d5e8895"
@@ -69,6 +75,16 @@
   revision = "012701e8669671499fc43e9792335a1dcbfe2afb"
 
 [[projects]]
+  branch = "master"
+  name = "github.com/bep/go-tocss"
+  packages = [
+    "scss",
+    "scss/libsass",
+    "tocss"
+  ]
+  revision = "2abb118dc8688b6c7df44e12f4152c2bded9b19c"
+
+[[projects]]
   name = "github.com/chaseadamsio/goorgeous"
   packages = ["."]
   revision = "dcf1ef873b8987bf12596fe6951c48347986eb2f"
@@ -109,6 +125,12 @@
 
 [[projects]]
   branch = "master"
+  name = "github.com/dsnet/golib"
+  packages = ["memfile"]
+  revision = "1ea1667757804fdcccc5a1810e09aba618885ac2"
+
+[[projects]]
+  branch = "master"
   name = "github.com/eknkc/amber"
   packages = [
     ".",
@@ -233,6 +255,12 @@
 
 [[projects]]
   branch = "master"
+  name = "github.com/mitchellh/hashstructure"
+  packages = ["."]
+  revision = "2bca23e0e452137f789efbc8610126fd8b94f73b"
+
+[[projects]]
+  branch = "master"
   name = "github.com/mitchellh/mapstructure"
   packages = ["."]
   revision = "00c29f56e2386353d58c599509e8dc3801b0d716"
@@ -356,6 +384,42 @@
   version = "v1.2.1"
 
 [[projects]]
+  name = "github.com/tdewolff/minify"
+  packages = [
+    ".",
+    "css",
+    "html",
+    "js",
+    "json",
+    "svg",
+    "xml"
+  ]
+  revision = "8d72a4127ae33b755e95bffede9b92e396267ce2"
+  version = "v2.3.5"
+
+[[projects]]
+  name = "github.com/tdewolff/parse"
+  packages = [
+    ".",
+    "buffer",
+    "css",
+    "html",
+    "js",
+    "json",
+    "strconv",
+    "svg",
+    "xml"
+  ]
+  revision = "d739d6fccb0971177e06352fea02d3552625efb1"
+  version = "v2.3.3"
+
+[[projects]]
+  branch = "master"
+  name = "github.com/wellington/go-libsass"
+  packages = ["libs"]
+  revision = "615eaa47ef794d037c1906a0eb7bf85375a5decf"
+
+[[projects]]
   name = "github.com/yosssi/ace"
   packages = ["."]
   revision = "ea038f4770b6746c3f8f84f14fa60d9fe1205b56"
@@ -431,6 +495,6 @@
 [solve-meta]
   analyzer-name = "dep"
   analyzer-version = 1
-  inputs-digest = "78b19539f7321429f217fc482de9e7cb4e2edd9b054ba8ec36b1e62bc4281b4f"
+  inputs-digest = "aaf909f54ae33c5a70f692e19e59834106bcbbe5d16724ff3998907734e32c0b"
   solver-name = "gps-cdcl"
   solver-version = 1
diff --git a/Gopkg.toml b/Gopkg.toml
index c87b82823..8e6a614f2 100644
--- a/Gopkg.toml
+++ b/Gopkg.toml
@@ -17,6 +17,14 @@
   name = "github.com/bep/gitmap"
   
 [[constraint]]
+   branch = "master"
+   name = "github.com/bep/go-tocss"
+
+[[override]]
+   branch = "master"
+   name = "github.com/wellington/go-libsass"
+ 
+[[constraint]]
  name = "github.com/chaseadamsio/goorgeous"
  version = "^1.1.0"
 
@@ -149,3 +157,15 @@
 [[constraint]]
   name = "github.com/bep/debounce"
   version = "^1.1.0"
+
+[[constraint]]
+  name = "gith