Catch invalid maildirs more gracefully
Use fatal() for BUGs
Use BYTES meta-variable in usage help text
Don't show an unsolicited diff when size difference exceeds threshold
Add --dry-run for testing removal
Add another removal strategy: --remove-matching

With --remove-matching=REGEXP, all duplicates whose filenames
match REGEXP are removed.  If all duplicates within a set match,
then none are removed; this prevents overly loose regular
expressions accidentally causing data loss.
Add --remove-not-matching=REGEXP

This works just like --remove-matching, except with the logic negated.