Feature request: Joystick support

It would be great to be using the nubs of the Open Pandora device, which can appear to the system as independent high-res joysticks.

For this reason, I have done the work. :) Please see the pull request: https://github.com/antirez/load81/pull/39

Up to 4 independent joystick devices can be used within LOAD81. For now, button support is not complete, but it should definitely be added as soon as possible (there are no 'joystick buttons' on the Pandora - all buttons are normal keyboard buttons, include A/B/X/Y, and triggers).

Work on adding joystick buttons (and other joystick features, such as 'hats' and 'trackballs' type inputs) should be completed.