Add ctrl+click hyperlinks for Haskell identifiers and URLs in .hs files

This implements #117, URLs are also working, no matter where they are. There are some edge cases where the behavior is a bit funny, e.g. when you have `test = "http://google.com"` then the URL detector and the Haskell detector are competing with each other when hovering the URL (the first one wins). This could be fixed by ignoring such things as strings completely, but this must be fixed in the regular "Open Declaration" action, as I make use of that code.

It works quite well, also very fast. I noticed though that sometimes the buildwrapper process isn't long-running (after certain actions, don't know) and then each call to thingatpoint spawns a new process which gets really slow. But this is unrelated to this pull request and should be fixed in general.