Add support for stylesheets

Despite it being named run-js, it would be nice if stylesheets would be automatically added if they exist. As well, support for processors like SASS and LESS would be really nice. Maybe regular CSS could be run through cssnext as well, like how regular JS is run through Babel right now.

There'd have to be a new definition of handlers (probably just add a type attribute to the file, see GH-2), and for those type of handlers, they'd just have a function that returns the compiled CSS. As well, the correct content type would need to be set. Probably wouldn't be too hard to implement, and would provide great value!