kubectl delete command: adding  labelSelector

@lavalamp @smarterclayton basically I've rewritten the PR integrating your comments.

@smarterclayton: I didn't fix shortcut expansion since I have hard time to understand where the shortcut should come from. Maybe we may discuss about it. 
After looking at what "all" returns there are a bunch of unsolved problems with all that we don't have to do right now (it returns all the resources in the system, including those that have no rest representation, or which we really don't want to delete, like nodes).  Let's just drop "all" and mapper.AllResources() and as a follow on figure out how to do it well.