Slim template lexer

I whipped up a Slim template lexer, only to discover there was a pull request for one already.... >_<

Anyway, this one has mostly complete support for the slim language, including splats and external parsers. Most of it is shamelessly ripped from the haml lexer. 
This is my first experience with regex, so there could be a lot of sub-optimal regexes here. Feel free to shout at me.

Also, feel free to just grab bits of my code and fix the already-open pull request.