RFC: Add ghc-modi syntax plugin for haskell

ghc-modi is a version of ghc-mod which caches data between runs, improving the time to check for errors to under half a second, even for large projects.

This is still more of an RFC, because I am not sure if this should go as a new checker or if I should merge the code directly into ghc-mod.  Also, is the change to SyntasticCheck ok?  Also, I need to add a command to kill the ghc-modi instance.