Support compact format file configuration

Allows the following configuration to in-place compress a single file:

```json
{"cssmin": {
	"compress": {
		"src": ["styles/app.css"]
	}
}}
```

Note: This does not support in-place minification of multiple files. More refactoring would be needed for that. IMHO this task should not do concatenation at all, as there is grunt-contrib-concat to do that. (Or at least make it optional?)