Added support for PubSubHubbub hub

This is useful for feeds that want to advertise PubSubHubbub support (see [PubSubHubbub discovery](http://pubsubhubbub.googlecode.com/svn/trunk/pubsubhubbub-core-0.4.html#discovery)). The update adds a new feed option `hub` that (if present) will be output as `<atom:link rel="hub" href="hub value"/>`