Added scope support for polymorphic association field

[DavidGeismarLtd]

At the moment associated models in a polymorphic association cannot be scoped, by default :all is called in candidate_resources_for(klass). This can produce pretty long lists in the select field.

• Added option to pass a scope to associated models in Polymorphic::Field
• Updated the example app to show an example with Customer
• Was not sure how to writte a test for that, open to suggestions.

First time contributing to open source here, open to any feedback on how to make better contributions 😃 .