Feature Request: durationSelector for handling time

[leblancmeneses]

A lot of operators dealing with time support durationSelector debounce/throttleTime.

Practical usecase: This feature might benefit others needing to integrate this in react-native when the app is in the background.

This allows other libraries responsible for time to be integrated with rxjs. Example: https://github.com/ocetnik/react-native-background-timer.