Condense `signal_add` call
Condense `signal_add` call

Instead of using the hash syntax to add our `message public` signal
listener, use the parameterised subroutine style call. Since we're only
adding a single signal, we don't really need to use the hash syntax.

I started out with it because I was following the example on
http://juerd.nl/site.plp/irssiscripttut

teddywing/irssi-hipchat-stfu,teddywing/irssi-hipchat-stfu