Add vote-based command invocation

[Zirak]

Some commands, like /ban and /die, are limited to owner-only, so that if a user starts abusing the bot and nobody's around, other users can't do anything about it.

Add support for vote-based invocation of some privileged commands. When 2 (or more) users invoke privileged command with the same parameters, it'll be executed.

A "vote" will be cleared after 10 minutes (10? maybe more?)