run an event when a notification starts

There is currently an `.onClose` method that can be setup to run when a notification closes, after looking up the docs and the code I've found no evidence for an opposite event that would fire when a notification is spawned.

My use case is that I want to play a sound when a notification is shown. So such an event would need to be defined at `.config` and would need to be run before any similar event would be run by a notification instance.

Is there a solution for this without poking inside the code and patching it ? if not I would probably fork the code and do it in the weekend.