> Alternate idea: instead of having formatters implement notifications, we document that their initialize method must accept a notifications arg and they must register the list of notifications they support with it. The purpose of this notifications business is to provide a way for formatters to tell us something, and the normal way ruby objects tell another object something is to send a message (i.e. call a method), so I think the semantics of having them call a method from initialize lines up with what we're trying to do, anyway.

That's an alternative to declaring notifications at all, but doesn't solve the "must be declared in your formatter" issue, as you'd still have to implement `def initialize` yourself to let us know you're compliant. Also note this is an additional breaking change, as currently `initialize` is left open field to developers.