Support JSON feed version 1.1

[Kiwow]

Closes #246

Thanks for your hard work maintaining this! I had a go at implementing support for the JSON feed version 1.1 spec.

I've added:

• support for multiple authors for both feeds and items. In the case that a single author is provided, we convert it into a 1-item array and use it in the "authors" field, because the spec has deprecated "author" and recommends only using the array field.
• support for the "language" field for both feeds and items
• declaration of the 1.1 feed version
• tests that fail without this PR and succeed with it
• workaround for multiple feed authors for RSS and Atom - to implement multiple feed authors for JSON feed, I changed the type of FeedOptions.author to include an array of authors. When multiple feed authors are provided, I've made it so that only the first one is used in case of RSS and Atom. This could be addressed in the future, although I'm not sure about how/if RSS and Atom support multiple feed authors

I'm not sure if provided contributors should be merged into the authors array, but I haven't done so yet. In the (current) version 1 implementation, contributors are ignored for JSON feed as well. If it makes sense to you to merge contributors in, I don't have a problem with adding that bit.