Allow setting no_ping and if sending a ping send it of the right type

[mitchcapper]

the kind:ping causes slack to constantly respond with an invalid message type now. Also allow disabling the ping in case the user wants to handle it on their own.

[FlyveHest]

Plus one, I updated SlackBots and suddenly started getting error messages all over.

I've rolled my own ping a long time ago, so being able to disable the built-in ping would be really nice.