Add Stack ghc feature

[johnwalker]

I'd really like to be able to use stack ghc instead of cabal repl. To do this, we just need to work inside haskell-process.el, right?

[gracjan]

There is a thing called process-type and it currently supports cabal-repl and ghci. It should be pretty easy to add stack ghc to the list of supported methods.

[johnwalker]

OK, I'll put in a PR today.