Add traverseCollect to TraverseFilter typeclass

[LukaJCB]

Analogous to the collect method on FunctorFilter we can define a traverseCollect method derived from traverseFilter using a PartialFunction like this:

 def traverseCollect[F[_]: TraverseFilter, G[_]: Applicative, A, B](fa: F[A], f: PartialFunction[A, G[B]]): G[F[B]] =
    fa.traverseFilter(a => f.lift(a).sequence)

[emilhotkowski]

Can I help with this? I'm looking for my first contribution to Cats.

[armanbilge]

@emilhotkowski indeed, that would be great!