add ghc-mod

Actually I am not maintainer of `ghc-mod` (or any other package I added), but `ghc-mod` is really great package, so I thought that it will be good idea to add it to Stackage.
I checked all dependencies of every package I am adding, and everything is already on Stackage. But I haven't builded it on my machine, because I got stuck at `./dist/build/stackage/stackage build`. Just hope that everything will work as it should.