Add ISO 3166-1 validation

[hkwu]

We can add a validator for country codes: https://en.wikipedia.org/wiki/ISO_3166-1.

[Haroenv]

I’d like to have this for language codes too, but I don’t see a way of providing this without immensely bloating the library. Maybe just the format and not if the language actually exists?

Never mind, that’s #572

[chriso]

See #734 for 2 letter country code validation.

3 letter country code validation is incoming: #809