Make it possible to use UserAssigned codes

[zeffron]

Problem

ISO-3166 reserves certain codes for user assignment. These are not included in the enumeration, so are unavailable with this crate. It would be useful to be able to use them.

Solution

Any solution works, but it seems like adding a UserAssigned variant to the CountryCode that takes a struct might be the best option.

[zeffron]

This would be helpful for working with datasets that include XK for Kosovo, which is not an official assignment, but is in common usage.