Support avatar in item author

[sn3h]

Is your feature request related to a problem? Please describe. There's no way to add article's author avatar to JSON feed.

Describe the solution you'd like Author only accpets name, email, link now, but should respect options from https://www.jsonfeed.org/version/1.1/.

Describe alternatives you've considered Creating separate feed where author's icon would be item's image, but there's problem with multiple authors anyway