Prioritise most recent senders when tab-completing nicks for reply

[LukeMoll]

If there are two users joel and joseph, jo tab-completes to joel, as it is alphabetically closer. However, if joseph sent a message more recently than joel, then the user is more likely to be replying to joseph than joel, so joseph should be preferred for completion. (Perhaps this scope should be limited to the last n messages or only those within m pages of backlog)

[sim642]

WeeChat's irc plugin has the option irc.look.nick_completion_smart for this. Similar behavior could just exist here, considering this is implemented at plugin level, not in WeeChat's core unfortunately.

[bqv]

Closing this as implemented in 58d46c7, whose bug is tracked by #13 instead