[feature request] `set` with relative arguments to file timestamp

[skymatr1x]

plus/add and minus/subtract timestamp of specified file attribute, with various units: day, hr, min, sec etc.

examples with file.txt existing mtime being "2001-01-01T01:01:01"

# changed to: "2001-01-03T01:01:01"
mactime set file.txt -m plus 2day

# changed to: "2001-01-01T00:01:01"
mactime set file.txt -m minus 1hr

# changed to: "2001-01-02T13:01:01"
mactime set file.txt -m plus 36hr

# changed to: "2001-01-01T01:31:01"
mactime set file.txt -m plus 30min

# changed to: "2001-01-01T01:00:51"
mactime set file.txt -m minus 10sec