[server_time] sneak in server-time extension.

[server_time] sneak in server-time extension.

This script implements IRCv3's server-time extension for most users'
purposes. For example, ZNC users can use this plugin to get clean
playback buffers.

I've updated this script to drop a dependency I had to download from
CPAN when I started using it. I did not initially create this script,
but I can provide support going forward.

I plan to implement the server-time extension in irssi's core myself
eventually, so maintaining this script seems fitting.