code: function done(output, options, config, failures, exit) {
    return marge.create(output, options)
      .then(([ htmlFile, jsonFile ]) => {
        if (!htmlFile && !jsonFile) {
          log('No files were generated', 'warn', config);
        } else {
          jsonFile && log(`Report JSON saved to ${jsonFile}`, null, config);
          htmlFile && log(`Report HTML saved to ${htmlFile}`, null, config);
        }
      })
      .catch(err => {
        log(err, 'error', config);
      })
      .then(() => {
        exit && exit(failures > 0 ? 1 : 0);
      });
  }
docs: Done function gets called before mocha exits
  
  Creates and saves the report HTML and JSON files
  
  @param {Object} output    Final report object
  @param {Object} options   Options to pass to report generator
  @param {Object} config    Reporter config object
  @param {Number} failures  Number of reported failures
  @param {Function} exit
  
  @return {Promise} Resolves with successful report creation