Support delegation to other ZeroName sites to bypass namecoin's limit on subdomains.

Namecoin allows a max of 520 bytes on transcation size,
that allows upto 12 subdomains to be recorderd in the blockchain
(assuming on average a subdomain has 1 character).

For clearnet, namecoin record can point to a nameserver record,
thus allowing unlimited subdomain.
But ZeroNet does not allow this yet.

Thus I suggest a new field `zeroname`, e.g.

```json
// example.bit
{
"zeronet": {
"": "root_domain_zeronet_address",
"foo": "subdomain_address"
},
"zeroname": {
"*": "ZeroNameFoo_zeronet_address"
}
```

So when zeronet encounter a domain `foo.example.bit`,
it resolves it according to ZeroName as usual.
And when zeronet encounter a domain `bar.example.bit`,
it cannot find a record in ZeroName, but find the delegation record to `ZeroNameFoo`.
So it resolves `bar.example.bit` via `ZeroNameFoo`.