Add IRC user authentication methods to command API.

[sgielen]

14400f9 adds permissions support: you can set permissions in scopes, ask if certain users have certain permissions, all via the plugin interface.

Three things are left for this issue:

• Add a plugin for setting and requesting permissions at runtime via IRC,
• Think about how to bootstrap this, e.g. define super-admins per network,
• Implement setting permission requirements when subscribing to a command, so that command will only be signaled if the user sending it has the needed permissions.