Adding iptables_nslookup()

This allows people to use something like:
```
      $dns_info = iptables_dns_info('www.gmail.com')
      
      firewall { "openvpn_${name}_${proto}_${port}":
        destination     => $dns_info['ip_v4'],
        destination_v6  => $dns_info['ip_v6'],
        protocol        => $proto,
        port            => $port,
        action          => 'allow',
        direction       => 'output',
        enable          => $bool_enable and $dns_info['enable_v4'],
        enable_v6       => $bool_enable and $dns_info['enable_v6'],
      }
```

If proto=udp, port=1194, then this results in:

  # cat /etc/iptables/rules.v4 | grep 1194
  -A OUTPUT --protocol udp --destination-port 1194 --destination 173.194.65.83 --source 0.0.0.0/0 -j ACCEPT 
  -A OUTPUT --protocol udp --destination-port 1194 --destination 173.194.65.19 --source 0.0.0.0/0 -j ACCEPT 
  -A OUTPUT --protocol udp --destination-port 1194 --destination 173.194.65.18 --source 0.0.0.0/0 -j ACCEPT 
  -A OUTPUT --protocol udp --destination-port 1194 --destination 173.194.65.17 --source 0.0.0.0/0 -j ACCEPT 

  # cat /etc/iptables/rules.v6 | grep 1194
  -A OUTPUT --protocol udp --destination-port 1194 --destination 2a00:1450:4013:c00::12 -j ACCEPT