yyyy/mm/dd date format

[idpaterson]

A user in Japan has noticed that his locale's customary yyyy/mm/dd format is not supported. This issue was briefly discussed in #70 with a potential solution at least when the year has 4 digits that would detect the yyyy/mm/dd format based on the position of the 4-digit sequence, regardless of the current locale's preferred date format. I think that's reasonable. Installing ICU didn't seem to help because this was skipped by the RE_DATE regex.

[idpaterson]

According to Wikipedia, only Kazakh uses yyyy.dd.mm, all other national date formats beginning with yyyy are in the order yyyy.mm.dd. I realized that ISO 8601 standard format yyyy-mm-dd is not matched either, so I will handle that case too.

It seems like the yyyy/mm/dd format should have worked for this user when ICU was installed, I'm not quite sure why it did not.