improve definition of custom selector

as noted by @achambers in #14 

> Maybe what the real change needed here is a better way to define custom selectors....At the moment you either use the defaults or override the jsonBlueprint property, which means you'd need to duplicate the current default selectors and add your new ones in.
> Maybe we can look at a way to say, I want the default selectors and also these other custom selectors. 
