Joystick support

[yurkobb]

First, thanks for developing this. It seems to match perfectly what I want to do for an (art) project. I wonder though, wouldn't it be nice to expose joystick controls similar to (mouse-x/y)? Maybe something like (joystick-axis axis), (joystick-btn btn)?

Edit: or shorter names, with the ability to select active joystick: (joyaxis [joystick 0] axis) / (joybtn [joystick 0] btn).