Add timestamp to (certain) output lines

[BenKaufmann]

Sometimes it is helpful to see when (relative to the start) the solver produced a result (e.g. a model).

While one could use something like ts to timestamp all output (e.g. https://unix.stackexchange.com/a/26797), it might be nice to have an option to enable (more fine-grained) printing of timestamps directly from the solver.

[MaxOstrowski]

So often I dreamed for this feature.
Whenever you have a logfile but you haven't done any precautions to time things this would allow to get a rough guess of how long things took (first model, grounding, all models). Especially if the process is still running.