Pass custom plugins to use

Hi there, 

I've got a custom rework plugin that I require() in my Gruntfile, and couldn't figure out how to pass it properly to grunt-rework. This patch allows you to just configure the plugin in the Gruntfile and pass it directly to Rework like so:

```
var reworkNamespace = require('rework-namespace-css')

...

use: [
	reworkNamespace({
		selector: '.wfa',
	})
],

```

If this functionality already exists then feel free to ignore, and maybe let me know the preferred method.

Thanks