Colorize nicks of people who left the channel before the export in the messages

[tobast]

At the moment, the nick coloration inside the message body is performed only on nicks that are still present in the channel.

• It would be possible to scan every message in the exported chunk to search for joins and parts before the actual processing, but this would probably slow down the export significantly the export on large logs
• It would also be possible to add nicks to the matched nicks on the fly, but that would mean recompiling a regex for each join/part of a new nick, which would possibly be also slow.