ISO 3166-2 countryCodes must be uppercase

[defacto64]

In light of the discussion we find on two incidents on https://bugzilla.mozilla.org/, I think than by now it is quite clear that the ISO 3166-2 countryCode must be in uppercase, at least from the point of view of some Root Programs.

At the moment Zlint ignores the countryCode case, so I think it would be appropriate to remedy this shortcoming. We can do this in two alternative ways: by modifying the already existing lint (e_subject_country_not_iso) or by introducing a new lint; what do you think is the best choice?

[christopher-henderson]

I think I'm somewhat partial to making a new lint. e_subject_country_not_iso checks for the presence of a country code, whereas what we are looking at here is whether-or-not the country code that is present is formatted correctly.