Add some basic clojurescript support

[dhleong]

The first call is a bit slow so I didn't set up a default BufRead autocmd. Also, it's unfortunate but most of the useful functions needed for full highlighting (ns-aliases, ns-resolve) aren't available in clojurescript. Also, since macros are compile-time only in clojurescript, we can't detect those at all.

So, I didn't disable the _core_keywords, but instead just augmented it.

Refs #5