Support for optional user-hashed names

[decentralgabe]

Consider an additional DNS property for an aka field in the DID Document. This could be a user-hashed name.

The user could 'mine' for a hash that extends their did (e.g. did:dht:example becomes did:dht:example:name). To add this property to their DID document they would find an associated nonce value that hashes to did:dht:example:name and include it in the DNS packet record.

[decentralgabe]

Make sure the alias is user-entered but also a URN. Could look into standardizing this.

[decentralgabe]

@mistermoe unless you think otherwise I am deprioritizing this to a later version of the spec. I believe it can be added as a non breaking extension and not crucial for 1.0.