add rss/atom feed support

[raffaelj]

It's not that hard to implement and I can't really call something a "blog" when I'm not able to read it with my feed reader.

Possible solutions:

• Finish the Feed addon
• or implement a simple xml output for feeds like I did with the sitemap

Resources:

• https://github.com/raffaelj/cockpit_Feed
• https://github.com/raffaelj/CpMultiplane/blob/master/modules/Multiplane/experimental/sitemap.php