Add cli option to start servo in fullscreen mode

[Mattia98]

Just like the title says.
It would be extremely helpful for kiosk applications or similar to be able to start servo in fullscreen mode. An argument like --fullscreen would suffice.
Usage case could be for display-only signage displays, where servo is started in fullscreen mode to display a certain webpage. Or perhaps for some kiosk application where a user is able to use the touchscreen to navigate a simple webui.
In any case, it would be useful to have a --fullscreen option.

[Quackdoc]

One kiosk application use case I look forward to eventually using is TV apps, example being youtube TV with the correct user agent