Add Warning Sound

[yamanq]

Hi, thanks for the great app. I think it would be nice to have a warning chime at 10 seconds and a countdown at 5 to 0 seconds. Otherwise, the clock runs out before we have a chance to reset it.

[Roccovdberg]

Hi! My name is Rocco, a student from the Netherlands, for a school project I need to contribute to an open source project, may I add this feature?