Short keypresses should move character

[qbolec]

First of all, I'd like to thank you so much for this project - as a ~35yo father of ~3yo son, I found your implementation as the only one available online which does not require cumbersome VM setup, buying actual device, watching advertisments or remaping keyboard. It's perfect for teaching my son logical thinking, intuitions about operations planing using motor skills etc. Big thanks! :)

Now, the actuall bug is that is that if my son presses a key for a short period, it might happen that the game logic loop does not even notice it.
I guess this is because https://github.com/jakesgordon/javascript-boulderdash/blob/master/boulderdash.js#L788 is performed synchronously instead of being scheduled for the next game loop iteration or something like that.
This is a bit problematic for a child with underdeveloped motor skills, beacuase pushing a button for too long causes a character too move to far, and pressing it once doesn't do anything.