Unused checks should ignore values annotated @unused

[ashleymercer]

Firstly - thanks for this library! It's saved me a lot of time :)

One very minor thing that would be helpful: the existing checks for UnusedConstructorParams and UnusedTypeParams could probably ignore values which have @scala.annotation.unused.

[ashleymercer]

I can try and put together a PR for this if you'd like?