function getExtPrefix(country, metadata) {
	metadata = new Metadata(metadata)
	if (metadata.hasCountry(country)) {
		return metadata.country(country).ext()
	}
	return DEFAULT_EXT_PREFIX
}
Returns extension prefix for a country.
@param  {string} country
@param  {object} metadata
@return {string?}
@example
// Returns " ext. "
getExtPrefix("US")