Support Stylus

[Stylus](http://learnboost.github.io/stylus/) is one of the popular stylesheet languages and not a few Bower components are written in Stylus, such as [colors](https://github.com/mrmrs/colors), [jeet](https://github.com/mojotech/jeet), [bootstrap-stylus](https://github.com/Acquisio/bootstrap-stylus), etc.

Actually we can use ".styl" files by extending `fileTypes` option, but I think it's more helpful to support Stylus officially.

Thanks.