[Feature request] Add ability to register a custom handler

Howdy folks 👋

I've seen a few people suggest custom handlers in issues, and some of them are good ideas but I can understand the reluctance to add more and more entries to a [global switch case that has all possible handlers](https://github.com/mixonic/ember-cli-deprecation-workflow/blob/master/vendor/ember-cli-deprecation-workflow/main.js#L41).

I was wondering if you were open to a modification to that code path that allowed you to specify a customHandler so that we could implement our weird and wonderful ideas as either a simple function passed in as the `handler` value, or some other way that might allow you `registerDeprecationHandler` in an addon so that it would work automatically?

I can see a few ways that I might implement something like this but I thought it was better to ask before spending any time on it 👍