adding support for cssproc in `app.yui.plugin()`

`cssproc` is a configuration object that can be used with shifter to update the path to each assets in any yui css module to make it work with combo and without combo, otherwise the relative paths to images in your css will not be resolved correctly.

The way it works is by doing this:

```
locatorObj.plug(app.yui.plugin({
    cssproc: true
}));
```

With that option, any time we build a module using `shifter`, we pass the `base` value from the group definition.

`shifter` uses `cssproc` module to do this process.

Todo:

* [x] docs
* [x] tests
* [x] example