Haskell-language-server: Generalise the formatter plugin infrastructure

[alanz]

We now have one formatter implemented, Ormolu via #33

This has been done in a "make it work" way, and I merged it to get concrete progress into the project.

We need to extract some sort of formatter plugin support from this, so it is easy to plug in other formatters.

[alanz]

I am going to add floskell, and see what ends up common

[Ailrun]

If there's no one working on stylish-haskell plugin, I will make that.

[Ailrun]

PR has been made: #146

[jneira]

Afaik there is a common formatter module with reusable functions for all formatters: https://github.com/haskell/haskell-language-server/blob/master/hls-plugin-api/src/Ide/Plugin/Formatter.hs
It can be extended if we discover more shared functionality