Add a 'desktop' subcommand

[docwhat]

It would be cool to have a 'desktop' subcommand that opened the correct Microsoft Remote Desktop application.

You can do this by constructing an rdp_uri and passing it to the correct program.

An example rdp_uri: rdp://full%20address=s:10.0.0.15&username=s:Administrator

You then pass this to the proper program on the OS:

• OS X: Use the open command.
• Linux: Use the xdg-open command.
• Windows: Pass it to Explorer.

It could use the same winrm_user and winrm_password values. It would make bug #169 all that more useful too.