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);
    }
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