Implement irssi parsing/formatting

[tilpner]

WIP in formats/irssi

[ailin-nemui]

keep in mind that irssi can be completely themed so possibly at best you can support the default.theme log format

[tilpner]

Yes, of course, I don't plan to support customization of logs at all, although it would be possible.
The current weechat parser relies on the standard date format as well, but... really, why would you even change that? :/