Add a true fullscreen option.

[artasparks]

Currently, Glift has hacks to create a new div and stop scrolling to emulate fullscreen.

Ideally, it should be using the native fullscreen implementation: https://developer.mozilla.org/en-US/docs/Web/Guide/API/DOM/Using_full_screen_mode

Consider this a provisional feature. We'll still need to support the hacks because the fullscreen api isn't supported on all browsers.