Add preliminary support for ghc-modi

This enables the `check` and `info` functions to use `ghc-modi` if it is available.

I had sketched an implementation for `make` as well, but it fails a lot of `ghcmod-vim`'s tests due to kazu-yamamoto/ghc-mod#275, so it's been left out for now.

Also, ghc-modi does not support spaces in filenames (kazu-yamamoto/ghc-mod#295), so `check` and `info` inherit this issue.

The `g:ghcmod_should_use_ghc_modi` flag determines whether the plugin attempts to use `ghc-modi`.