feat: enable visualizers to use p5's keyPressed

[liammulh]

In #120, @katestange was wanting to enable visualizers to use keyPressed.

p5 allows a user to define a keyPressed function. From the docs:

The keyPressed() function is called once every time a key is pressed.

The developer never calls the keyPressed function. Seemingly p5 does that. I imagine p5 looks at your source code, finds the keyPressed function you defined, and then uses it as a callback for a key down event listener. See this example.

I have no idea how to define the keyPressed function in a visualizer class so that it is called by p5.

[gwhitney]

This was enabled in #277. Closing.