Message tag support

[ilbelkyr]

There are various IRCv3 extensions that would be useful but require message tags, which we currently don't support.

We'll still need to decide on various specifics before working on the implementation, but I'm opening this issue for now to keep track of this and to reference it as dependency from other issues.

[donpdonp]

According to the release notes for charybdis 4, released March 2016, IRCv3 message tags are already implemented. https://github.com/charybdis-ircd/charybdis/blob/master/NEWS.md

Is the freenode fork different enough that it needs to be reimplemented or can it be merged from whats upstream?