Add reference type to repeater options

[JamiesonRoberts]

It would be good to be able to add a list of direct references via a repeater.

Scenario: You create a secondary content type called artwork. You need to attach multiple pieces of art to a post type of gallery. This way you can directly query via looping over the repeater field to get all artwork attached to a gallery dynamically. This also then lets you attach that artwork to multiple galleries, lets say a third type of "art show".

This allows for more complex data hierarchy where you can define reusable posts that can be attached multiple ways.

[MA2153]

Possibly blocked by #386