Add support for sitting out / leaving a game

[gmmeyer]

People should be able to stop the game. It shouldn't require restarting the app to stop the game.

Also, people should be able to leave the game, so that they don't hold up the game when they don't respond.

[CharlieHess]

From the directions:

To end a game, @<your-bot-name>: quit game

Agreed that players should be able to leave, definitely a good enhancement.