code: function addDirStats(results) {
          var dirStats = {
              all: 0,
              killed: 0,
              survived: 0,
              ignored: 0,
              untested: 0
          };
  
          _.forOwn(results, function(result) {
              var stats = result.stats || addDirStats(result).stats;
              dirStats.all += stats.all;
              dirStats.killed += stats.killed;
              dirStats.survived += stats.survived;
              dirStats.ignored += stats.ignored;
              dirStats.untested += stats.untested;
          });
  
          results.stats = dirStats;
          return results;
      }
docs: Decorate the results object with the stats for each directory.
  
  @param {object} results - (part of) mutation testing results
  @returns {object} - Mutation test results decorated with stats