Added syntax scope definition and completion for GHC extensions

This adds completions for GHC extensions, as of GHC 7.8.3. These are generated by `ghc --supported-languages`.

I also added a scope to the syntax definition to allow highlighting if desired, and to try to restrict the scope of the completions a bit, but I'm not really sure that worked. Nonetheless, I've left it in because it's such a small change.