Notification when aborted -> retried transition completes

For regular transitions, using the `then` method on the returned `Transition` works well. When the transition is completed then callback is called.

With aborted + retried transitions it's trickier. Since `retry` creates a new transition (code below), it isn't possible to get notified when it resolves.

1. Is this by design?
2. If not, would it be possible to improve this?

Copying over the transition would work, but may break things (I don't know). Another option would be to introduce another method, something like `retryableThen()` on `Promise`, whose underlying promise would get copied over to retried transitions.

What are your thoughts? (cc: @asakusuma @chadhietala)

```js
retry() {
this.abort();
let newTransition = this.router.transitionByIntent(this.intent as OpaqueIntent, false);

// [...] shortened for clarity
return newTransition;
}
```
Source: https://github.com/tildeio/router.js/blob/c82c0ca1b7/lib/router/transition.ts#L273

Start with lib/router/transition.ts at retry(), then trace router.transitionByIntent and the existing Transition.then behavior described in the issue. Determine how an aborted transition's retry could preserve completion notification semantics, and document the intended API and completion behavior before attempting an implementation.

Written by the indexing model from the issue text.