Full IRCv3 extensions support

[jpnurmi]

http://ircv3.atheme.org/

v3.1 base extensions

• capability negotiation
• multi-prefix
• sasl

  v3.1 optional extensions

• account-notify
• away-notify
• extended-join
• tls

  v3.2 base extensions

• message tags

  v3.2 optional extensions

• server-time (communi-shared/zncmanager)
• metadata

[jpnurmi]

IRCv3 is a never-ending story. Support for any particular extensions will be implemented upon need/request.