Provide integration point for other apps

[alcinnz]

If you register support handling application/xml+rss and application/xml+atom URIs, it will be very convenient for people to add webfeeds from Odysseus. Liferea and QtRSS already supports this, as will vocal in a future release.

I'm a bit busy right now, but I'll be happy to contribute a fix after next week.

P.S. I was looking for a nice feedreader for elementary OS, so thanks for this app!