add server-time support

[Thumpxr]

implement "server-time" capability so znc (bouncer) buffered text will be displayed with the right timestamps.

[Mikaela]

Noticing that this is missing wasn't very nice surprise when I opened holoirc. I thought all modern clients support this.

• Specification: https://github.com/ircv3/ircv3-specifications/blob/master/extensions/server-time-3.2.md

The ZNC capability is named znc.in/server-time-iso.

[dgw]

@Mikaela I've come across maybe two clients that support it. Granted I'm not trying new clients very often any more, but it's not really that widespread yet.

Still would be nice to have.

[spinza]

This is useful when buffer gets played back. Would be great.

[Mikaela]

The ZNC capability is named znc.in/server-time-iso.

In git version it's server-time since IRCv3.2 is released. https://github.com/znc/znc/pull/951

[spinza]

An explanation about server time: http://blog.alteroot.org/articles/2013-09-03/irc-server-time.html