code: function mkdir(dir) {
    return new Promise((resolve, reject) => {
      fs.lstat(dir, (err, stats) => {
        // Resolve, when already is directory
        if (!err) return stats.isDirectory() ? resolve() : reject(new Error(`${dir} is not a direcotry.`));
        // Try to create directory
        fs.mkdir(dir, (err) => {
          err ? reject(err) : resolve();
        });
      })
    })
  }
docs: Creates a direcotry or ensures that it exists.
  
  @param {string} dir The path where a directory should exist.
  @return {Promise<void>} Resolves when the given directory is existing.