Add DAC support (-A) for go-sqlcmd

[stuartpa]

[diego-deriv]

Hi this was supported by the mssql-tools/bin/sqlcmd, it will be really helpful to have this feature also for go-sqlcmd

[shueybubbles]

Diego Ramírez (@diego-deriv) if you are using the standard DAC port on SQL server I think you can just use localhost,1434 as the server name.
To support non-default ports/instance names, we'll need to augment the go-mssqldb driver to support the admin protocol.
I opened microsoft/go-mssqldb#108