[Engine-IRC] Support usermask channel modes +I and +e.

Add support for the usermask channel modes +I (user may enter an +i channel
without being explicitly invited) and +e (exception from the ban list).

This adds the commands /banexcept, /unbanexcept, /inviteexcept and
/uninviteexcept.
Merge pull request #54 from RavuAlHemio/experiments/more_mask_modes