There should be a more efficient way for plugins to check a user's prefix modes in a channel, without having to resort to hacks like https://github.com/GLolol/PyLink/blob/f25c260/plugins/relay.py#L161

More specifically, the IrcChannel class should implement functions like the following:

• isOp()
• isHalfop()
• isVoice()
• isAdmin()
• isOwner()
• getPrefixModes() (returns a joined string of all prefix modes the user has)

And as possible future extensions:

• isOpPlus
• isHalfopPlus
• isVoicePlus
• canSpeak (checks whether target matches any bans; whether channel is +m and user isn't voiced, and a variety of other situations)
• canChangeTopic (equals isHalfopPlus if channel is +t, or canSpeak otherwise)