Add support for minification of final output

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

hugolib/testhelpers_test.go

@@ -130,6 +130,7 @@ func (s *sitesBuilder) WithConfigTemplate(data interface{}, format, configTempla
 func (s *sitesBuilder) WithViper(v *viper.Viper) *sitesBuilder {
 	loadDefaultSettingsFor(v)
 	s.Cfg = v
+
 	return s
 }