code: function readKeys(force) {
      if (!force && options.cacheKeys) {
        return Promise.resolve(cachedKeys);
      }
      if (!force && options.cacheValues) {
        return Promise.resolve(Object.keys(cachedData));
      }
  
      return readdir(file).then(function(files) {
        var keys = [];
  
        files.forEach(function(filename) {
          if (filename.endsWith('.json')) {
            keys.push(filename.replace(/\.json$/, ''));
          }
        });
  
        return Promise.resolve(keys);
      });
    }
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