Custom entries/extensions support may need added

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

```json
{
"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)**

```yaml
custom_entries:
- entry_name
```

[link__json_feed__spec__extensions]: https://jsonfeed.org/version/1#extensions

Start by locating how feed.json FrontMatter is read and how JSON Feed items are generated; the issue proposes a custom_entries configuration. Check the JSON Feed extensions specification linked in the issue, and consider the work complete when configured underscore-prefixed custom objects are emitted in items without affecting standard entries.

Written by the indexing model from the issue text.