Return cached canvas instead of null when requesting canvas multiple times

This pull request adds caching for canvas get requests. This simplifies cases where we want to request the canvas dynamically. For example when the program draws something on a paper placed at the whisker.

Example:

const whisker = await paper.get('whisker', { direction: 'up' });

var canvas;
var ctx;

whisker.on('paperAdded', ({paperNumber}) => {
   // without the caching this would work only once 
  // each subsequent request would return null  instead of the canvas
   canvas = paper.get('canvas', { number: paperNumber }) 
   ctx = canvas.getContext('2d')
})

whisker.on('paperRemoved', () => {
  canvas = null;
  ctx = null;
});

setInterval(async () => {
    if (canvas) {
     // draw something on other canvas
    }
})

janpaul123 / paperprograms

Return cached canvas instead of null when requesting canvas multiple times #66