Add support for IRCv3 capabilities

[xPaw]

http://ircv3.net/irc/

Would be nice to at least support server-time at first.

Comparison of clients and what capabilities they support: http://ircv3.net/software/clients.html

[flyinprogrammer]

We probably want SASL support too so that launching clients in AWS and GCP is a little easier:
https://github.com/charybdis-ircd/charybdis/blob/master/doc/sasl.txt

I barely know how to code in JS, but I'll see if I can get you a PR -- no promises though.

[Pumpuli]

I tried adding support for message tags in slate-irc-parser, but never went further than that,