Adds the passed proc as a listener on the channel indicated by
channel_name. listener_key is used to remove the listener later; if none is
specified, the proc itself is used.
Whatever is used as the listener_key is returned, making it very easy to
use the proc itself as the listener_key:
listener = add_listener("Channel") { ... }
remove_listener("Channel", listener)
Calls all the procs registered on the channel indicated by channel_name. If
value is specified, it is passed in to the procs, otherwise they are called
with no arguments.