add RSS feed output format

[aaronpk]

I'd love to be able to use Granary to generate a podcast feed from an h-feed instead of having to do that myself! Some examples that would make great podcast feeds:

• https://martymcgui.re/tag/this-week-indieweb-podcast/
• https://aaronparecki.com/tag/100daysofmusic

There are a few requirements for podcast feeds beyond traditional RSS feeds. It's a bit of a trick to get it right. If this sounds like it'd be doable I'd happily help document everything needed and how it could be parsed from the h-feed.

[snarfed]

hoo boy, whole new format! agreed, RSS would be a great addition.

i'm unlikely to work on it myself anytime soon, but i'd happily support anyone who wants to! adding a new format to granary is actually pretty fun and self contained, much more so than adding a whole new silo (since there's no auth etc). JSON Feed is a good example, see jsonfeed.py and #107.

[jgmac1106]

+1 to this. I do wonder about the p-category and Apple requiring you to use their specific tags.

[snarfed]

shipped! feel free to try, feedback welcome.