Add label matching according to RFC 1123

[kiarashazarnia]

This feature could be helpful to check the RFC 1123 Label standard.

One of the major use-cases is checking strings to be valid for Kubernetes Namespace.

Thanks for your attention. I hope it will be helpful for you folks.

[swaeberle]

I think this would be also helpful when validating a single hostname (without any domain) for RFC1123 conformance.