Support for echo-message capability

[ShaneMcC]

(http://ircv3.net/specs/extensions/echo-message-3.2.html)

Rudimentary support for echo-message would be nice. Should be pretty straight-forward from a client point of view. If the Parser has the echo-message capability enabled, then don't display our own PRIVMSG/NOTICE events, and instead wait for the server to send them to us.

[ShaneMcC]

We probably want to do this by having the parser firing events when messages are sent, and then just not firing them until the right time.