Pass options to GHC to support cabal sandboxes (using optparse-applicative).

I have the ```safe``` package installed in a sandbox, not in the
main user cabal directory, so this will fail:

    $ haskell-docs Safe findJust

With my commit we can pass the package database options using ```-g```:

    $ haskell-docs -g '-no-user-package-db' -g '-package-db /home/carlo/foo/.cabal-sandbox/x86_64-linux-ghc-7.6.3-packages.conf.d' Safe findJust
    findJust :: forall a. (a -> Bool) -> [a] -> a
    findJust op = fromJust . find op