Add the ".cljx" file extension to the list of Clojure languages.

Some people start writing portable Clojure/Clojurescript code and use
the ".cljx" file extension for that. This is driven by this project:

https://github.com/lynaghk/cljx
Merge pull request #669 from r0man/cljx

Add the ".cljx" file extension to the list of Clojure languages.