Add a quiet output flag (-q)

[ghost]

I think this could be useful for people who use this cli app on scripts, like me. Thank you for this app too, btw.

[Daijobou]

+1
A reason: When you use PHP and call cli with shell_execute() I must add 2>&1 1> /dev/null after command, otherwise an warning message will be logged in php error.log on each call.
WARNING: [pool example.com] child 1721038 said into stderr: "vkEnumeratePhysicalDevices failed -3"
A quiet flag could simplify it :)