Support additional matching mechanisms

[slurmulon]

It's been a while since I worked on this, but ran into some issues on personal projects that could benefit from fancy PubSub.

One of the main annoyances I find is only being able to match with json-where. It would be nice if we also supported the following matching mechanisms:

• Arbitrary String
• JSON Schema

This will require refactoring the matching functionality a bit (obviously), most likely involves adding a new layer of abstraction so that we can disambiguate what sort of matching pattern is applied (JSON Schema, json-where, exact string, etc.)