Countries with ISO codes

[JoshuaBehrens]

Is it beneficial and match this project's goals to add more quality to the data? Looking up countries by their name is hasslesome. For better working with the data I'd like to see some alpha3 codes based on ISO 3166-1.

[jpwhite3]

In general I'm a big fan of improving data quality, structure, and testability. That said, I started the codebase to use as a teaching tool in a classroom environment and I'm sensitive that other may be using it for the same purposes. So I want to be careful not to introduce breaking changes without conversation.

Adding a column for ISO codes sounds like a reasonable new feature that shouldn't break existing functionality, so I'm open to it.