Added support for ClojureScript extensions to --clojure

Added support for ClojureScript (.cljs) and Leniningen's cljx cross-compiler (.cljx) to --clojure option.

I ran ```make test``` and everything passed.

I also considered breaking Clojurescript into a separate option, but they're still fairly tied together (e.g., Clojurescript macros are still written in Clojure, likely to have back-ends written in Clojure, etc) so I figured this was more useful.

Let me know if you have any questions!