function optimizeFolder(config, dir, output) {
  if (!config.quiet) {
    console.log(`Processing directory '${dir}':\n`);
  }
  return fs.promises
    .readdir(dir)
    .then((files) => processDirectory(config, dir, files, output));
}
Optimize SVG files in a directory.

@param {any} config options
@param {string} dir input directory
@param {string} output output directory
@return {Promise<any>}