code: function preparePlugins(output) {
      var book = output.getBook();
  
      return Promise()
  
      // Only load plugins for main book
      .then(function() {
          if (book.isLanguageBook()) {
              return output.getPlugins();
          } else {
              return Plugins.loadForBook(book);
          }
      })
  
      // Update book's configuration using the plugins
      .then(function(plugins) {
          return Plugins.validateConfig(book, plugins)
          .then(function(newBook) {
              return output.merge({
                  book: newBook,
                  plugins: plugins
              });
          });
      });
  }
docs: Load and setup plugins
  
  @param {Output}
  @return {Promise<Output>}