adapt to json_feed_model Item struct

[hdhoang]

hi! thanks for making this. i'm trying this out as an atom/rss -> filtering/enriching -> json feed aggregator.

The custom data format is nearly compatible with JSON Feed item. The json_feed_model crate at bluk/json-feed-model provides convenient get/set and (feed-level) validation of the schema.

How do you think about changing the serialization format for custom sink, or a new jsonfeed sink which use those field name?

This would lessen the downstream scripts' work in renaming them. In particular, non-URL item ID are not sent via custom output yet.

cheers!