SelectorButton exposed to select just countries

[rsegecin]

I believe it would be beneficial for many Apps that also require Addresses from users to use the same assets provided by your library when selecting the country. So this pull request only exposes SelectorButton widget along with ContryProvider and CountryModel enabling intl_phone_number_input to work just as a country selector. It also adds a boolean variable "showName" to the SelectorConfig so it will show country names while hiding "isoCode".

Best regards.

[Galti]

Yes, I also would like to get that functionality. Otherwise I have to use different package for selecting countries.