Support JSON Feed version 1.1

[przemoc]

Requested Feature: Support JSON Feed version 1.1

Changes in 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.

Related Area: Plugin

Do you want to contribute this yourself as a pull request?

• Yes, I have already written code for it (link if available and feasible)
• Yes, I don’t have code ready yet (that’s okay!)
• No (that’s okay too!)

Does this feature affect backwards compatibility? If yes, in what way?
It replaces author field (object) with authors field (array of objects).
For backwards compatibility I believe both could be provided.

Rationale and full description: (why should it be added to Nikola?)
Currently jsonfeed plugin supports only version 1.0.
It's better to support latest version of the standard, i.e. 1.1 (which is 1 year old already) for better interoperability in the ecosystem.

[Kwpolska]

Those changes were fairly minor and easy to implement, so I went ahead and did them.

(I moved the issue to the plugin repo, since it fits better here.)

[przemoc]

Sorry for misplacing the issue in main repo instead of plugins repo.
Thank you for addressing the request so quickly!