Add highlighting usage

Close #9.

This PR isn't ready for merge. I open it early for discussion.

Highlighting usage currently works for `*.fs` files in an active project.
A few things need to be done:
 - [ ] Add non-blocking GUI
 - [ ] Refactor `HighlightUsageTagger` to be more readable
 - [ ] Probably allow to highlight on a `*.fs` file without any project. FSharp.Compiler.Service supports that.
 - [ ] Fix bugs and consistencies (I noticed some but didn't investigate them further)

However, the biggest issue is to access F# Project System. I did implement by trial and error. I managed to get references and source files, but couldn't get compiler arguments properly. It would be nice to understand F# Project System (it has no documentation at all). and update the implementation.