Bug fix in parsing date for --onlyShow option.

Zero-padding was not being enforced by the library (dateparser) which was being used to parse the date passed to --onlyShow. This is fixed now to enforce zero padding in the month, days, minutes & seconds. Eg. Before this fix --onlyShow since 200731 would have listed sequences with modification dates after March 1 2007, instead of July 31, 2020, which it NOW correctly does, as 200731 should be interprted as YY MM DD, and not YYYY M D as the dateparser library was doing.

Also, slightly better --help output was added.