Implement pragma handling

@danedaworld and I implemented `{-# LANGUAGE ... #-}` pragma handling.

Somebody more experienced than us should probably look at how we handle extensions that are not found in [Parser.hs](https://github.com/razzius/IHaskell/blob/allow_pragmas/src/IHaskell/Eval/Parser.hs).

Before:

After:

Closes #309.