Adding capability to delete resources via label selector

@smarterclayton, @bgrant0607  @derekwaynecarr Thanks to have a look when you have time
My understanding is that we may want to run: kubectl delete pods,services, replicationcontrollers -l ... (a space between comma after services but no space between comma after pods and services"
In this case you may want -> "pods", "services", "replicationcontrollers"