Added support for alpha-3 keys

Perhaps I shouldn't actually have opened an issue ticket first. Ah well, so this is linked to #3442. 

Does not break any code, as the `alpha3` config value defaults to `false` which leaves the functionality as it was. 

This is incomplete however, as I can't figure out how to actually make config values work outside of the class!... I can't figure out how to set `$this->config()->alpha3`, but clearly the config values must be set somewhere else. I just can't find where; it's not in this class file since those other config variables (default_to_locale and default_country) can't actually be changed by changing them in this file. Someone enlighten me?