#215 Per channel log filename customization (log rotation)

[Nemo]

Currently it's possible to customize logs filenames (and directories) to have a log rotation, but only on a global basis: this is not very useful since different channels have different traffic, and it doesn't make sense to split in the same way the logs of a channel disregarding if they're 4 MB/month or 20 KB/month.
It would be very useful to be able to set this preference separately for each channel (at least withouth a GUI), as it's possible e.g. on Irssi (see http://irssi.org/documentation/startup#c8 , although it's not clearly explained here).