Add support for country flags

[sreuter]

Awesome work! Any special reason why country flags are not supported?

[tomekkleszcz]

Hi, This is an oversight on my part. I will implement the country flags in my free time. :)

[sreuter]

Awesome, we should be able to piggyback on https://github.com/twitter/twemoji-parser/blob/master/src/lib/regex.js I assume. It doesn't work as a direct replacement for your current regex tho, as reactStringReplace seems to want it in a different format.