Implement the censor module

This is the new implementation of the former BADWORD command. The command has been changed to CENSOR to be more consistent with other IRCd implementations and it is now enforced in the form of a channel mode (+G).
Can this be named after CENSOR?  And maybe also change the letter to make more sense (maybe 'C' for CENSOR since 'G' for mode consistency is already used by g:lines).