dddd returning as valid V6 address

```
require('ip').isV6Format('dddd')
```

I checked on http://www.helpsystems.com/intermapper/ipv6-test-address-validation and it's invalid according to that. and also the inet_pton function http://locutus.io/php/network/inet_pton/ throws an error with `dddd`.

So it seems like the isV6Format function of node-ip has a bug.

Reproduce the issue with require('ip').isV6Format('dddd'), then inspect the isV6Format entry point and its address-validation logic. Compare the result with the referenced IPv6 validator and inet_pton behavior; done means the invalid input is rejected without breaking valid IPv6 address checks.

Written by the indexing model from the issue text.