Pomodoro response should mention creator

[suchow]

Right now, when The Reverend announces that a pomodoro is over, he uses plain text but does not mention any users, so that nobody gets a notification about it. He should @mention people so that they get a ping.

[suchow]

It would also be good to allow multiple people to be added to a pomodoro, e.g.,

reverend, start pomodoro with jack and jill