Ported privmsg privacy option from growl-net

If xmpp_reveal_privmsg is set to OFF, then the notification
will only state the nick name and not the content of the message.

Ported from https://github.com/kfdm/irssi-growl
Ported topic notifications from growl-net

If xmpp_show_topic is set to ON, then notifications
for topic changes will be sent.

Ported from https://github.com/kfdm/irssi-growl