Add support for multiple rss feed urls

[thomasdziedzic]

I am planning on using pacmatic to display news from archlinux.org and a separate test server which makes sure that a basic installation boots successfully on our device after automatic updates.

If you're not interested in implementing this functionality, would you be interested in accepting a pull request for such a feature? I'm still not sure this will be our final solution, but I'm playing around with ideas :)