Reasonably format used time in TAP output

[fnogatz]

Since version 0.5.0 library(tap) prints the used overall time at the end of the run tests as part of the summary:

# time=1000.3ms
# tests 4
# pass  4

It would be nice to add a new predicate for usage in tap_footer/3 that nicely formats this duration and its unit, e.g., to print 16.3s instead of 16312.6ms, 2:03min instead of 120312.6ms, etc.