Swift 5.0 update

Added the prefill method:

let source = NKVSource(countryCode: "ru")
textField.setCurrentCountryInitially = false
textField.preFillText(source: source, number: 7999432423)

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Thanks @shahbaz-saleem for pushing me to update to swift 5.0 😁
Thanks @artemy-ant for some features 🙌