Q module: Add join-on-invite option.

Use case: On QuakeNet you can /msg Q CHANLEV #chan <yourself> +j to
be invited once you authenticate. Anyone known on the channel (mode +k)
is able to set/unset chanlev +j for themselves.

Signed-off-by: Anders Bergh <anders1@gmail.com>
Merge pull request #524 from anders/q-invite

Q module: Add join-on-invite option.