matter.js: piano action

:steam_locomotive: I think the [upright piano](http://codepen.io/sharethewisdom/full/RKRVrL/) action may be good
for a challenge. Also see the [grand piano](http://codepen.io/sharethewisdom/full/GrWoXb/) action.

My disorderly attempts to animate svg parts using
GSAP (see www.greensock.com), turn out being a
bit heavy on the CPU.

Thanks to the CodingTrain I got to know matter.js!
This is perfect, and I'm eager to completely rethink
my attempts. I'd like to have a physical model, and:

* render on canvas (GPU)
* maybe move away from svg
* not hard code the collisions, velocities etc
* trigger a sound when the hammer collides with
the string until the damper mutes it
* provide parameters (sliders!) to:
- adjust screws
- bend the metal rod of the catcher
(on the back of the key lever)
- change the tension of springs

I'll post back here if I got a working attempt.
cheers! Bart