Update voxel-fly - allow crouching while in flight. Closes GH-36

deathcap/voxel-fly@489f2145c9ad41591215eebb05b32cdcde822106 Shift+space to crouch while not ascending nor descending
deathcap/voxel-fly@dd640b9232371876fd7fb4788ac8e24e57715461 Change to use kb-bindings instead of adding new document.body DOM listeners

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