commands.join-tabexistingchannels will toggle tab completion for channels that
the user is already in. Defaults to false as most users will just use /join
for joining, not for trying to focus existing windows.
Fixes issue 4092
Change-Id: Ie5c260b202ff8617a28edb90838e4b6a57b7f326
Reviewed-on: http://gerrit.dmdirc.com/1201
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Gregory Holmes <greg@dmdirc.com>