add date format ISO8601_DATE (YYYY-MM-DD)

Currently DATE pattern grabs months and days first. When encountering an ISO8601 date of YYYY-MM-DD, it will take the first 2 digit as the month or day, off-setting the rest of the date. 

So I've added the ISO8601_DATE and try to have that match first to the DATE pattern. This is more for ease of use since it will let users to just throw DATESTAMP pattern and have it properly match the year, month, and day without having to create their own patterns (or use the long TIMESTAMP_ISO8601 format)