anchor: Joystick:getGamepadMappingString
  Gets the full gamepad mapping string of this Joystick, or nil if it's not recognized as a gamepad.
  
  The mapping string contains binding information used to map the Joystick's buttons an axes to the standard gamepad layout, and can be used later with love.joystick.loadGamepadMappings.
  mappingstring = Joystick:getGamepadMappingString()
  mappingstring string A string containing the Joystick's gamepad mappings, or nil if the Joystick is not recognized as a gamepad.