Support a decreasing timer by configuration

[dvberkel]

Pull-request #596 introduces a new implementation for the timer. It allows for a custom formatter. With this formatter one can implement a decreasing timer.

Unfortunately this requires to provide custom code, which is error prone and maybe not everybody that wants to use remark is capable to provide it.

This is a request for a new feature: allow to configure the timer to be decreasing with certain target time.

[dvberkel]

If this this feature is deemed interesting, I am more than willing to implement it.