Add Support for JSON Feed v1

[benmccormick]

This PR adds support for v1 of JSON Feed: https://jsonfeed.org/

It's still pretty early days for JSON Feed but feed readers are starting to support it and it was fairly straightforward to add it as an option for feed.

Things to note from this PR:

1. I added 2 new top level options, favicon and feedLinks. The JSON feed spec heavily encourages including a link to the feed url and specifying a separate favicon and site image. I couldn't find analogues to these things in RSS2.0, but Atom supports both, so I made the changes to atom so setting a favicon and an image will work for both atom and JSON Feed. For feedLinks, there was an existing options.feed that seemingly assumed that it was an atom feed link. Since a user might presumably want to generate both an atom and a JSON feed (since JSON feed will have limited support for a while), I felt it was necessary to add a new config object that could take both JSON and atom feed links. I maintained the existing behavior though, and the atom link will get pulled from options.feed if its available, but fallback options.feedLinks.atom. If you're on board with this, a deprecation message could possibly be added for options.feed.

2. Not all concepts that feed supports are handled by JSON feed. Contributor lists, multiple authors, and "categories" don't map cleanly to JSON Feed. I ignored most of those, but clipped authors down to the first author if multiple were given.

3. I fixed a typo in the README and added examples for the new fields

4. I parsed to JSON before testing the JSON feed output, because Jest will give nice diffs if JSON objects are different that it wouldn't be smart enough to do right if a string comparison were wrong.

I'm happy to clean up anything you're concerned about here, just let me know what you think.

[coveralls]

Coverage increased (+1.3%) to 89.503% when pulling a0c835e on benmccormick:master into 2810732 on jpmonette:master.

[coveralls]

Coverage increased (+1.3%) to 89.503% when pulling f1ce790 on benmccormick:master into b68d4cc on jpmonette:master.

[jpmonette]

Thanks for your work @benmccormick - just merged things in the repo.

In case you do another PR in the future, make sure to split it in multiple commits for each of the points you enumerated above. It would be easier to link the change with a description instead of all changes in one commit.

[benmccormick]

@jpmonette great thanks for merging! I will keep that in mind for the future.