stylish-haskell configuration file (EXPERIMENTAL, don't merge yet)

[marcosh]

address partially #100

My proposal is to use https://hackage.haskell.org/package/hlint and https://hackage.haskell.org/package/stylish-haskell. Generally they should be installed locally and integrated with the development environment (for sublime-text, see https://packagecontrol.io/packages/SublimeLinter-hlint and https://packagecontrol.io/packages/StylishHaskell).

They are not tools which make you forget about styling, since they do not touch too much the structure of the code. If we want something more invasive, we could look at hindent, but I must say that I don't really like how it formats the code

It would probably be nice to add them to the CI process, but I really have no idea how to add them to Nix

[epost]

Thanks for picking this up Marco! I've been experimenting with these myself as well. Let's leave this PR as experimental just for a little bit, as I'd like to have a closer look at all of the settings and perhaps throw in some other formatters and possibly CI integration too.