Add parameter to select Xbox or PS4 mappings

[nstiurca]

This implements feature request #76

[nstiurca]

I could only test this locally with PS4 controller, so someone needs to verify that everything still works OK with Xbox controller.

[jack-oquin]

Looks good to me after a cursory review. Thanks for defining constants for the controls.

Let us know when it's been tested with the other controller.

[nstiurca]

I got my hands on an Xbox controller, and it seems to be working all right.

It looks like on Xbox controller, it uses the right thumbstick left/right for turning. I was using left thumb stick for both speed and turn on the PS4 joystick bindings. I guess I should change it for consistency?

[jack-oquin]

I'm not up to speed on those devices. Do whatever seems best to you.

Thanks!

[nstiurca]

Ok, then I would leave this PR it as-is. For whoever was already using this node, there should be no behavioral change whatsoever. For me, I have the exact same functionality with controls slightly better suited to my liking.

[jack-oquin]

Sure. If anyone wants the other behavior, we can add it as an option.