Custom entries/extensions support may need added

[S0AndS0]

According to JSON Feed -- Spec -- Extensions "items" may contain custom objects, provided that the key name is prefixed by an underscore, eg...

{
  "items": [
    {
      "_entry_name": {
        "about": "http://example.com/json-feed-extensions/",
        "something": "value"
      }
    }
  ]
}

Likely this feature should be added as FrontMatter configuration for feed.json files, for example something like...

feed.json (snip)

custom_entries:
  - entry_name