code: function writeCollections(drizzleData) {
    return Promise.all(walkCollections(drizzleData.patterns, drizzleData)).then(
      writePromises => drizzleData,
      error => DrizzleError.error(error, drizzleData.options.debug)
    );
  }
docs: Traverse the patterns object and write out collections HTML pages.
  
  @param {Object} drizzleData
  @return {Promise} resolving to {Object} drizzleData