Add support for author in the feed plugin

[oscarotero]

Enter your suggestions in details:

Useful for blogs with multiple authors.

• It's a top-level value and a per-item value.
• It must contain name and url subfields, so this requires to include the authorName and authorUrl fields.

Specs:

• JSON Feed: https://www.jsonfeed.org/version/1/
• Atom: https://validator.w3.org/feed/docs/atom.html