Add options for simple elapsed time printing

[hollasch]

One option is to specify the modification age of a file. Equivalent to

timeprint --modification <file> --now ...

A second improvement would be a simple, smart elapsed time format code. This would be in the format

%_Y, %_yD, %_Dh:%_h0M:%_m0S

In practice it would be a bit smarter than that, eliding large units with zero value. For example, if the total elapsed time was under a day, then elide years and days.