Move should stop walking logic into voxel-walk

Ref https://github.com/flyswatter/voxel-walk/issues/3
Remove shouldStopWalking option, always use built-in
Get controlsTarget and skin from game on enable, if not set in options

Allows for easily using with voxel-player, as long as voxel-walk loads after,
no need to pass skin or controlsTarget explicitly, as it can be loaded from
the game instance.
Add missing repository field to package.json

(to stop npm from complaining)
Update to use full plugin names

Requires deathcap/voxel-plugins@0a3117a8c4a1ce4b33d2f80271db8aefae4a841f