Edited implementing the country picker modal.

[kigh143]

Added a working description on how to implement the country picker modal.

1. Changed the functions selectCountry and onPressFlag to fat arrow function.
2. Changed the function body for onPressFlag setting the visibility of the modal to true instead of invoking the open function on the country picker reference.
3. Change the onChange function on the CountryPicker element to onSelect

[zizzle6717]

I have published v1.0.0 and converted the source to typescript. Let me know if there is anything I can do to improve the change request process. Please submit new pull requests as the package as been more to https://github.com/rili-live/react-native-phone-input