something like this

// main.js

var s = function(p) {
    p.setup = function() {
        p.createCanvas(768, 512);
        p.m = new mandelbrot(p.createImage(p.height, p.width));
    };

    p.draw = function() {
            ...
    };
};

var sk = new p5(s, mandelbrot_set);


// mandelbrot.js

function mandelbrot(image) {
   ...
      var c = color(56); // nope, cant do it
   ...
}