Basic joystick support

No support for balls and hats yet.
HTML5 Gamepad API requires to push button on the gamepad to set it to active state.
If you're using Firefox, open about:config and set dom.gamepad.enabled and dom.gamepad.non_standard_events.enabled to true to allow Firefox handle input from your gamepad.

Bug: button and axes events can be dispatched twice in some browsers.
Solves https://github.com/YellowAfterlife/openfl-bitfive/issues/13