Feature request - Timescale for the vcd files.

simavr is a great tool when it comes to check cycle  timings. 
However, at 8Mhz, 10ns is not sufficient to get integer timings. 
In my specific case I just edited sim_vcd_file.c, but adding a way of selecting the base (a command line option?) may be very helpful for this usage of the simulator.
