Better ISO 8601 parsing

[jakozaur]

There are several more valid date formats which this library does not support:
E.g. 2017-02 is a valid ISO 8601 date.

See:
https://en.wikipedia.org/wiki/ISO_8601
https://ijmacd.github.io/rfc3339-iso8601/

So that you know, this library has yet to support more valid date formats. See week dates or ordinal dates.

[relvacode]

You are right, the purpose of this library has always been to improve interop with other languages which don't exactly conform to RFC3339 with minimal overhead and avoid using extensive regexp patterns like other libraries. Perhaps that should be on the README.

Anyway, thanks. Happy to take more PRs if there's something else you need. Week and ordinal dates are likely to add a lot more complexity as they can't just be passed off to time.Date