Duel should check if the user is present

[marcog]

11:24 < WOAHPSSH> challenge accepted
11:24 < tibid> accepted: The gauntlet has been thrown at your feet. Do you accept?
11:25 < tibid> accepted refuses to meet your challenge and accepts dishonour

The duel plugin should check to see if the challenged user is present. When undirected, the bot should ignore the message. When directed, it should say "User not present" / "I don't know about user".

[Stefano Rivera (stefanor)]

Um it should also require you to address the bot, when you start a duel.

I'm less and less impressed with how we handle addressed=False