Syntax: %bHUB [hub server]%b

If [hub server] is given, displays a list of all the servers that
are using [hub server] as their hub. If [hub server] is not given,
displays a list of all current hub servers.
Examples:
         %bHUB irc.mainhub.com%b
           Shows all leaf servers using irc.mainhub.com as their
           hub.

         %bHUB%b
           Displays a list of all hub servers.