Add slash command to blackhole/scrub

[andrewyager]

From a chatops perspective, it would be wonderful to be able to trigger block and scrub actions via a slack slash command (an extension of the /ddos_blackhole or similar command) currently implemented to display the list of blackholes.

[andrewyager]

This feature should probably have the ability to have some security over it, preventing "any" slack user from triggering this type of behaviour.