Add support for RSS 2.0

[jaredcwhite]

The reason this plugin uses Atom is for historical reasons (it started as a fork of the jekyll-feed plugin). I don't have a particular dog in the now-ancient Atom vs. RSS fight, but I do like the idea of supporting RSS 2.0. It's really not much different from the outside looking in. Honestly I'd just love to refactor the whole thing and use a declarative component-based rendering pipeline where the exact XML tag output can easily change depending on the renderer.

[jaredcwhite]

Update: I looked into this some more, and using Ruby's RSS gem we get a nice declarative builder DSL: https://github.com/ruby/rss

So no more Liquid file required. My plan at this point is to switch entirely to RSS 2.0 in order to support #8 and also just simplify the code.

[michebble]

@jaredcwhite
I was wondering if this is still in progress. I recently migrated to Bridgetown and lost RSS 2.0 in the process, I was looking at rolling my own before I noticed this plugin and this issue. Though I noticed the rss-next branch is stale.

Could I possibly help with its development?

[jaredcwhite]

@michebble Yeah it's just on hold while I focus on core features for upcoming Bridgetown 2.0. This is a priority after that. I am using it on one of my sites in production, so feel free to load this branch in your gemfile and try it out. Happy to accept any PRs or issues filed as you run into potential problems. Hopefully it's in decent shape though.