Add contextual p5 support

Hello,

I typically run p5 myself and avoid adding p5 methods to the window object. That being said, p5.gif should have the option to use custom p5 contexts, such as:
```js
new p5(function(p5) {
  p.setup = function() {
    recorder = p5Gif.capture({
      p5: p5
    });
  };
  p.draw = function() {
    recorder.addFrame();
  };

}, 'cont');
```

Thanks
Robert