Support For One Object, Multiple Templates

I have a project that requires several looks for a pass based on various criteria. Maintaining several pass type ids, configurations and classes ends up being time consuming and bloated. This allows the object to determine what pass template to use based on the pass_type_id attribute value rather than the class name. I also added some test coverage to ensure backwards compatibility, and updated the README.md.

I thought this might be a useful feature for others too.