User preference for discarding two moves in computer vs. human game #99
Description
When the user discards the most recent move in a computer vs. human game, it is then the computer's turn to play. To be able to play, the user must then discard a second (her own) move.
There should be a user preference that, if turned on, causes a tap on the discard button to discard two moves. This would cause discard to behave very much like undo did in 0.9. The user preference should take effect only in a computer vs. human game.