docs: function lint (files) {
    const stream = gulp.src(files)
    .pipe($.plumber())
    .pipe($.eslint())
    .pipe($.eslint.format())
    if (process.env.NODE_ENV === 'production') {
      stream.pipe($.eslint.failOnError())
    }
    stream.on('error', onError)
    return stream
  }