[feature] Support JSON feed v1.1

[binyamin]

Is your feature request related to a problem? Please describe.
The JSON feed spec is currently at v1.1. The changes from v1.0 to v1.1 are very minor, so this should be a pretty easy box to check off.

Describe the solution you'd like
Here's a synopsis of changes, taken from the specification:

Version 1.1 — 8/7/2020:

• Updated to use more specific application/feed+json MIME type.
• Added authors field to allow more than 1 author per feed or item. Deprecated author.
• Added language field.
• Clarified that id should be a string.
• Updated links to JSON and other specs.

[hacdias]

I see @jlelse forked it and implemented it: https://github.com/jlelse/feeds. I see that there's barely any activity on this repo and some stale PRs so I don't feel this repository will be updated soon.

[stale]

This issue has been automatically marked as stale because it hasn't seen a recent update. It'll be automatically closed in a few days.

[geira]

I see JSON feed is a spec, but is it a community accepted standard? Has it been adopted by any recognized standards body, e.g. W3C? Although it is lacking many essenial feaures like official extension namespaces (e.g. Dublin Core) I'm sure it could be useful for many, but currently it only looks like a private initiative without much market traction.

[s3rj1k]

I see JSON feed is a spec, but is it a community accepted standard? Has it been adopted by any recognized standards body, e.g. W3C? Although it is lacking many essenial feaures like official extension namespaces (e.g. Dublin Core) I'm sure it could be useful for many, but currently it only looks like a private initiative without much market traction.

If so, support of version 1.0 should be dropped from package