Add gamepad controls

[darkf]

Using a gamepad/controller would be a useful improvement for people preferring that playstyle, and can work similarly to WASD movement (#94).

We can use the experimental Gamepad API for this (which works at least in Chrome and Firefox at the moment.)

[darkf]

Also, with analog controls, we can have 6-directional movement (corresponding to the hex directions), and add up/down (similar to W/D).