function getCountryCallingCode(country, metadata) {
	metadata = new Metadata(metadata)
	if (metadata.hasCountry(country)) {
		return metadata.country(country).countryCallingCode()
	}
	throw new Error(`Unknown country: ${country}`)
}
Returns "country calling code" for a country.
Throws an error if the country doesn't exist or isn't supported by this library.
@param  {string} country
@param  {object} metadata
@return {string}
@example
// Returns "44"
getCountryCallingCode("GB")