code: function validateConfig(book, plugins) {
      return Promise.reduce(plugins, function(newBook, plugin) {
          return validatePluginConfig(newBook, plugin);
      }, book);
  }
docs: Validate a book configuration for plugins and
  returns an update configuration with default values.
  
  @param {Book}
  @param {OrderedMap<String:Plugin>}
  @return {Promise<Book>}