Introduce strategy for use with strong_parameters

This works similarly to ActiveRecordWithEagerAttributesStrategy, but for
attributes to be assigned, you must provide the name of the method to be
called, via the :attributes option to the `expose` call.

The assumption here is you'll define a private method in the controller
and use strong parameters, as:

    expose(:foo, attributes: :foo_params)

    def foo_params
      params.require(:foo).permit(:bar, :baz)
    end

Once merged, this should close out #59