code: function writePages(drizzleData) {
    return Promise.all(walkPages(drizzleData.pages, drizzleData)).then(
      () => drizzleData,
      error => DrizzleError.error(error, drizzleData.options.debug)
    );
  }
docs: Write out HTML pages for pages data.
  
  @param {Object} drizzleData
  @return {Promise} resolving to drizzleData