Remove hard coded quit when Escape is pressed
**Is your feature request related to a problem? Please describe.**
I hope you will consider this small change - to remove hard coded quit when Escape is pressed - for the following reasons:

* Usually users expects Escape to toggle a menu. I'm currently working on some UI, but unfortunately using Escape is not an option right now.
* Game state (player stats, current level etc.) might get lost, because it's not possible to save before quitting.

**Describe the solution you'd like**
Remove hard coded quit completely and let developers use `pyxel.quit()`. 
 
**Describe alternatives you've considered**
* Allow overriding the default behavior.
* An option to disable the default behavior.

Thank you! :)