Hugo Pipes added minification support for resources fetched via ´resources.Get` and similar.
This also adds support for minification of the final output for supported output formats: HTML, XML, SVG, CSS, JavaScript, JSON.
To enable, run Hugo with the `--minify` flag:
```bash
hugo --minify
```
This commit is also a major spring cleaning of the `transform` package to allow the new minification step fit into that processing chain.
Fixes#1251
