docs: function processFile(dirMetas) {
    return (file, callback) => {
      const filePath = `.${path.sep}${file.path}`;
      const dirMeta = path.parse(filePath).dir.split(path.sep)
        .map((dirName, i, arr) => path.join(...arr.slice(0, i), dirName))
        .filter(dirPath => dirPath in dirMetas)
        .reduce((obj, dirPath) => Object.assign(obj, dirMetas[dirPath]), {});
      const meta = Object.assign({}, dirMeta, file.meta);
      async.nextTick(callback, null, Object.assign({}, file, { meta }));
    };
  }