[RxJS] backoff notifier handler for Observable.retryWhen()?

[huan]

Could we make a new interface for Rx.Observable.retryWhen handler?

Here's a backoff example in RxJS document:
https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/retrywhen.md#example-an-incremental-back-off-strategy-for-handling-errors