Allow admins and owners tracking via account.

This change allows tracking of admins and owners with accounts instead of hostnames. Hostnames still work.

Nothing else in the commands need to be changed, as the decorators themselves were altered to properly check for account. The `is_admin` command was modified to take this into account (no pun intended) and now requires a nickname to be fed (previously a hostname). While I was there, I also made `is_owner` (currently not used anywhere).

There was discussion a few minutes ago in -dev over whether or not !fallow and !fdeny should be modified regarding this change, and we agreed to leave those alone for now.

It's not currently possible to check for accounts if the user and the bot don't share at least a channel. I haven't found a sane way to do that.