Remove use of fmt.Sprintf for simple string concatenation. A simple
change for a small perf boost.
```
name old time/op new time/op delta
Integrity-4 525ns ± 2% 268ns ± 2% -48.92% (p=0.000 n=10+10)
name old alloc/op new alloc/op delta
Integrity-4 144B ± 0% 112B ± 0% -22.22% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
Integrity-4 5.00 ± 0% 3.00 ± 0% -40.00% (p=0.000 n=10+10)
```
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#4381Fixes#4903Fixes#4858
