Hyphens in domain label

Hello,

According to [RFC 1035](https://tools.ietf.org/html/rfc1035#section-2.3), domain label cannot start or end in hyphen. Validation on email addresses such as ```email("andy-noble@data-workshop.-com")``` should throw a ValidationFailure while the function currently returns True.