Add a way to make reference to a previous message of the backlog

[jonathanmarsaud]

Hi,

As discussed in #ircv3, it would be nice to have a feature which permit to jump to a message of the backlog instead of having to say "Can you please take a look at the message at 17:42 on this channel?". A simple way will be to have a standard parsing rule/format which parse every timestamps and by default, jump to this timestamp of the actual channel (it needs to be discussed how we can do the same if we make reference to another channel). As suggested on #ircv3, it can also be a metadata like @link={msgid}.

Regards,