Send `LINELEN` ISUPPORT or allow +draft/multiline

[4e554c4c]

Description

irslackd allows for more than 512 byte lines. Recently we received a feature request in halloy to support this. However, an IRC client should not send over 512 byte lines (to improve interoperability) unless the server indicates otherwise.

One way to indicate a longer supported line length is the LINELEN ISUPPORT parameter (see the horse docs). Another (better) way to do this is is to support the multiline ircv3 extension, which allows for sending longer messages using multiple commands