Add SSH tunneling support

[aaronjheng]

Add flags like those from MySQL Shell.

  --ssh=<value>                    Make SSH tunnel connection usingUniform
                                   Resource Identifier. Format:
                                   [user[:pass]@]host[:port]
  --ssh-identity-file=<file>       File from which the private key for public
                                   key authentication is read.

Related: gomodule/redigo#673