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. IMHO this task should not do concatenation at all, as there is grunt-contrib-concat to do that.