Add .duelon/.dueloff for users to prevent themselves from being dueled

[dgw]

Sometimes people just don't want to be dueled. Sometimes there are channel bots with whom users shouldn't be able to initiate duels.

BombBot has it, so why doesn't duel? (Yes, I've been thinking about doing this for some weeks now. And to be quite perfectly honest, I thought I'd already created an issue.)

[dgw]

Should have left the issue reference out until I was done --amending…