Add JSON Feed to RSS converter (Apple podcast support)

[bcomnes]

After going down the jsonfeed-to-atom path, I realized, despite Atom being a much cleaner format and serialization, apple chose side with RSS, so I implemented an RSS converter and included full apple podcast extension support. This should enable people to publish podcasts with JSON Feed and easily convert them into an Apple podcast compatible feed, perhaps helping justify going down the json feed route in the first place.

[bcomnes]

Possible to land this?

[manton]

Thanks!