Allow to define behaviours on `Countdown`

[Byloth]

Right now, countdown allows you to define different behaviour every time you start it.
This means that you need to define new behaviours every time you start it again.

Allow to define behaviours just a single time (on declaration or even after) and then use them every time you start it.
You'll still able to adds different behaviours on start calling.

Use the same then, catch, finally format as Promises.

[Byloth]

Maybe solved in d91c88f?