isValid function
Check if an IBAN is valid.
Implementation
bool isValid(String _iban, {bool sanitize = true}) {
var iban = sanitize ? _stripSpaces(_iban) : _iban;
if (iban.length < 2) return false;
var spec = specifications[iban.substring(0, 2)];
return spec != null && spec.isValid(iban);
}