Holding forward

[freeman-lab]

User testing (specifically my nieces) shows that people often hold down on the forward key instead of tapping it to move forward. As a result, they continue to move forward upon getting to the next tile. We should consider how to prevent this, either by changing what happens when you reach a tile (resetting?), or by allowing a movement mode in which on holding the key.

cc @sethvincent @sofroniewn