Add slim lexer

I whipped up a Slim template lexer, only to discover there was a pull request for one already.... >_<

Anyway, this one has complete support for the slim language, including splats and the whole shebang. Most of it is shamelessly ripped from the haml lexer. Tests are not added yet, will do later today.

This is my first experience with regex, so there could be a lot of sub-optimal regexes here. Feel free to shout at me.