Support for IRCv3

[msteltenpool]

I'm aware that this might be a big request, my use case currently is aimed at Twitch. More specifically the Capability Negotiation and Message Tags (http://ircv3.net/irc/), and possibly some others.

Currently I'm hacking these in to send a CAP REQ and filter out the message tags before parsing. I could try and add these as all the other commands are added, and make message tags be recognized by the parser. But not sure if adding in some tidbits of the IRCv3 is the way to go or if architectural changes are also needed to better handle this.