Add an experimental rssjs feed based on the experimental rss.js spec.

This is simply a JSON representation of the RSS 2.0 feed, accessible at /feed/rssjs/ anywhere.

props pento.
see #25639.


git-svn-id: https://develop.svn.wordpress.org/trunk@26294 602fd350-edb4-49c9-b593-d223f7449a82