Add support for JSON Feed (#388)

Add support for JSON Feed (#388)

* Add support for JSON Feed

Because JSON Feed is (obviously) not XML, I couldn't use SAXMachine to
construct the feed and entry objects, so I had to roll my own.  However,
because of methods depending on SAXMachine in Feedjira::Feed (in particular,
call_on_each_feed_entry), I still had to include SAXMachine in the JSON
Feed classes.

The API of the JSON Feed class should match up fairly closely with
the existing XML feed classes, although the actual JSON Feed
specification names some elements differently. I thought compatibility
and familiarity trump keeping the original names in the spec.

This does not support the attachments feature of the JSON Feed spec.
This may be added later.

Please see the spec for more details: https://jsonfeed.org/version/1