Allow users to invite bot into new channels

[gkatsev]

This also means we need a way to ask to leave. Together, it could be abused.
One possibility is to have the bot check to see whether the invite/exit is from a channel op before doing so.

[ljharb]

Perhaps, requiring it be from an op in a channel the bot is already in, would be useful.

[robotlolita]

Couldn't the bot just keep a whitelist of people who can execute "trusted actions" in it? Then we just need to add a command to add / remove people from this list.

[ljharb]

I like that idea, with or without this one.

[gkatsev]

Yep, trusted users is a separate, but related, feature.