Promise support for notifications

[TobiasGrether]

Description

Instead of just being able to provide a time in milliseconds after which the notification is completed, it would be great to have support to pass a promise to it.

Explanation / motivation

If you want to use reapop to show for example the status of a file upload or similar, you can't give an exact estimate in ms on when that operation will succeed

Additional information

• [ ] I can implement this feature/improvement

[msanandrea]

Same opened => https://github.com/LouisBarranqueiro/reapop/issues/526

[TobiasGrether]

See #526 for solution