Please add option to ignore timestamp differences in containers

This bug was originally reported by Felipe Sateler (fsateler@debian.org) in Debian bug #907600:

diffoscope has been a very useful tool for me, in comparing dumps of
various sorts spit out by several systems. It would be great to have an
option to ignore timestamps, in order to be able to check if two
tarballs/zipballs/whateverballs have the same contents, even if the
contents were generated on different dates.