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.

`shifter` uses `cssproc` module to do this process.

This new feature is blocked by: https://github.com/caridy/cssproc/pull/1

Todo:

* [ ] docs
* [ ] tests