Update loader "add" method to return self

[alexstandiford]

This would allow for method chaining, and could make registering things a little cleaner. Example:

underpin()->meta()->add( 'type_1', [ /*args*/])
underpin()->meta()->add( 'type_2', [ /*args*/])

vs

underpin()->meta()
          ->add( 'type_1', [ /*args*/])
          ->add( 'type_1', [ /*args*/])