Add auto-DNS support in tunnel.

Version: v6

Not supported DNS currently, for example `proxy1.com` and `proxy2.com`.
CMD:

```
go-dispatch-proxy.exe -lhost 127.0.0.1 -lport 5566 -tunnel proxy1.com:1234 proxy2.com:1234
```

User need to find ip of domain name.

Could you add auto-DNS support?


