Support @ to identify server in argument

[sharego]

Current, dnslookup uses fixed position for different arguments, first is domain, second is server, it likes nslookup command. However, sometime we forget this constraint.

The dig command use @ to set server, no position required, the we can use like this dnslookup @server domain [<providerName> <serverPk>] or dnslookup domain @server [<providerName> <serverPk>], or dnslookup domain server [<providerName> <serverPk>]