Add support to get the ISO-8601 numeric representation of the day of the week when using "N"

Hello, 

Since the [`getDay()`](https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Date/getDay) method provided by the javascript `Date` object does not follow the ISO 8601 and this library does it for the week number, so I though that will be a nice addition and would be useful.