Add ISO3166-3 support

[apainintheneck]

This is not currently provided by the countries gem and I doubt it will be added in the future though there is an issue open for it (countries/countries#538) but that hasn't been updated in five years. ISO3166-3 is a list of countries that used to be included in ISO3166-1 but were removed for some reason.

If we were to add it, it seems obvious to me that it should be lower priority than any of the other country matchers (see Atlasq::Data.country). We'd also have to see about collecting information about the countries in question so that the output is similar to what we show for the current crop of countries.

It would be nice to have though not essential IMO.