Change to use kb-bindings instead of adding new document.body DOM listeners
Shift+space to crouch while not ascending nor descending

Useful since crouching may be used for other actions you want to
do simultaneously while flying (such as voxel-voila detailed info,
or voxel-use place block on GUI-enabled block without opening GUI).

Closes https://github.com/deathcap/voxpopuli/issues/36