Add day as a new time unit option to the existing minute and hours.

[kitatek]

Rationale:

• oftentimes, I find that I need to interrupt a tasks days after I left it open: due to week-end or vacation or busy on other things.

• being able to say this task stopped 8 days ago is more productive rather than multiplying mentally by 24 to specify hours instead, making calculation mistakes on the way.