Add a `OnOccasionaly` event

[Edouard-chin]

Useful when you don't really know when the todo comes to expiration but want to be reminded every so often

[francois]

That would require to remember state from run to run. A stateless way would be to have a percentage chance of notifying, like 10% of runs, or approximately every once every 10 runs. It might be never, or it might be every run. It's a roll of the dice :)