Allow ISO 8601 timezone offset in `DateTime.fromString()`

[cedx]

The DateTime.fromString() method only allows ISO 8601 dates that have the following format:
YYYY-MM-DDThh:mm:ss[.SSS]Z

But it is very common to have dates that use a timezone offset, like 2020-04-24T16:20:07+02:00.

So it would be cool if the DateTime.fromString() method supported dates using a timezone offset.
Currently, it's very hard to deal with ISO 8601 dates in Haxe!