Support fallback nicks when nick is already in use

[SoniEx2]

Feature Description

It would be nice to be able to specify fallback nicks, instead of adding random digits at the end.

[mokahless]

Not sure if it's related enough or if I should start a new issue for this but:

I think related, I'd like to be able to specify no fallback. Sometimes, I will return to thelounge months later to find my username permanently changed in settings from "username" to "username8" etc. And obviously because of this, the servers with proper username lookups and logins just stop working.