code: function getRelativePath(filePath) {
      var urlPath = path.join(
        path.dirname(filePath),
        path.basename(filePath, path.extname(filePath))
      );
      if (options.slugify) {
        urlPath = urlPath.split('/').map(function(part) {
          return slug(part, options.slugOptions);
        }).join('/');
      }
      urlPath += !options.prettyUrls || (/(^|\/)index$/i).test(urlPath) ?
        '.html' : '/index.html';
      return urlPath;
    }
docs: transform a src file path into an relative url path
  @param  {string} filePath file path
  @return {string}          url