Feature request: supporting of the listening on ip:port for the input argument.

Hi,

I try the following command but failed:
```
$ dnsproxy -u tls://8.8.4.4 -u tls://8.8.8.8 -l 172.17.0.1:9999 -v
2020/11/02 11:40:48 [info] Starting the DNS proxy
2020/11/02 11:40:48 132327#1 [debug] Upstream 0: tls://8.8.4.4:853
2020/11/02 11:40:48 132327#1 [debug] Upstream 1: tls://8.8.8.8:853
2020/11/02 11:40:48 132327#1 [fatal] cannot parse 172.17.0.1:9999
```

I can't figure out why dnsproxy doesn't support the form of listening on ip:port as the input argument.

Any hints will be highly appreciated.

Regards,
HY