Pass options to GHC to support cabal sandboxes.

I have the ```safe``` package installed in a sandbox, not in the
main user cabal directory, so this fails as expected:

    $ haskell-docs getdoc Safe findJust
    Couldn't find that module. Suggestions are forthcoming.

With my commit we can pass the package database options using ```-g```
in a similar way to ghc-mod:

    $ haskell-docs getdoc -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 op = fromJust . find op