Added CAP identify-message for supported servers an as option

Library lacks any kind of real authentication measure -  you can't check if user is registered with nickserv without actually asking nickserv. That takes a while for response, and with extensive amount of querying it wouldn't realy work well. Fix I added doesn't break any current usability by default, but if you enable it you'll be able to get user's authorization status just by checking first message character (- if unathorized, + if authorized).