Add country enum support.

[kalkusa]

I believe it's a good idea to add support of country enum. Some country codes can be cumbersome to remember and I didn't see any information on what country codes are supported.

I've created an enum based on your countries.ts in the example directory. Although I don't think all of these country flags are supported, I believe you can filter out some of them.

I hope you'll find this improvement helpful.

[okinawaa]

cool..... This change save my life