Provide a way to reference named items from a BagPart

[sebastienros]

Let's say a BagPart of Images is attached to a BlogPost. The template will either iterate over each item, or will pick specific items. Right now you can only pick an item by index, it would be better to get one by a logical name. Either id or title.

[sebastienros]

The difference with pointing directly to an asset is that you can own custom fields on a content type. Then we need a new liquid tag (only liquid) to be able to reference one of these items from the body editor.

{% content_item_reference "MyBagPart["foo"] | render %}