code: function readKeys(force) {
      if (!force && options.cacheKeys) {
        return Promise.resolve(cachedKeys);
      }
  
      return readData().then(function(data) {
        try {
          return Promise.resolve(Object.keys(data));
        } catch(err) {
          return Promise.reject(err);
        }
      });
    }
docs: @private
  Reads the keys from the file
  
  @param {boolean} [force] Whether or not to force a reread of the data
  from the file
  
  @returns {Promise} A promise that resolves to an array of the keys