code: function downloadCanvas(canvas, filename=config.defaultFilename, extensionType='image/png') {
          let url = canvas.toDataURL(extensionType);
          let link = document.createElement('a');
  
          link.href = url;
          link.download = filename;
          document.body.appendChild(link);
          link.click();
          document.body.removeChild(link);
      }
docs: Triggers browser to download image, convert canvas to url,
  we need to append the link el to the dom before clicking it for Firefox to register
  point <a> at it and trigger click
  @param  {object} canvas TYPE: el <canvas>
  @param  {string} filename
  @param  {string} extensionType