Add Clojure support for .boot files

[Boot](http://boot-clj.com/) is a Clojure build tool that uses the `.boot` extension for Clojure code run in the Boot environment.  It would rock if `.boot` files were Clojure highlighted.
Right now we do list the "primary" extension for a language first, and then the rest in alphabetical order. Historically this had significance in the code, treating the primary extension differently than alternate extensions. It doesn't currently, but I think we should keep the convention of listing the primary extension first for now.