code: function relative(dir, file) {
      var isDirectory = file.slice(-1) === '/';
      return normalize(path.relative(dir, file)) + (isDirectory? '/': '');
  }
docs: Convert an absolute path to a relative path for a specific folder (dir)
  ('test/', 'hello.md') -> '../hello.md'
  
  @param {String} dir: current directory
  @param {String} file: absolute path of file
  @return {String}