Added JSON Feed (@jsonfeed) Format to the Universal feedparser Gem

Hello,

  I've added support for reading feeds in the new JSON Feed [1] format
in - surprise, surprise - JSON to the universal feedparser [2] gem.
Nothing changes :wink: Use it like:

    require 'open-uri'
    require 'feedparser'

    txt = open( https://jsonfeed.org/feed.json ).read

    feed = FeedParser::Parser.parse( txt )

    pp feed

  Enjoy. Cheers.

[1] https://jsonfeed.org
[2] https://github.com/feedparser/feedparser