in timelog, support an option to define ones own time format in strftime() format

[wwalker]

default strftime/date format (in ilts) is :

%Y-%m-%d_%H:%M:%S.%N
2019-12-06_15:26:40.740187

(micros is not supported in the strftime() format, ilts is written in Go which does support it)

The date command does support %N (nanoseconds)

This will depend/interoperate with the solution for #3