extend Backbone.RelationalModel?

I'd like to try Knockback, but need to use Backbone.RelationalModel. Seems like it depends on Backbone.ModelRef... Could Backbone.ModelRef extend [Backbone.RelationalModel](https://github.com/PaulUithol/Backbone-relational) (as an option)?

Start by reading the Backbone.ModelRef implementation and the linked Backbone.RelationalModel project to understand their model behavior and compatibility requirements. Determine the scope of making RelationalModel support optional, then define how the option would be enabled and how completion would be verified.

Written by the indexing model from the issue text.