Make joining channels optional (795864)

A request made against the debian package at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=795864

Quoting the original bug report

"However, if a channel is not mode +n (or whatever the local equivalent of this channel mode on a particular IRC network is), it's possible to send PRIVMSGs to channels without joining them.

This may be preferable for several reasons:

• no join/part messages at all

• no need to establish more than one connection to the irc server to be able to reach dozens of channels"

The original submitter suggests extending the JSON api to handle this case, so it is exposed to the hook scripts.

This seems something that may be useful.