code: function transpile() {
          return rollup.rollup({
              entry: path.join(dirs.source, files.main)
          }).then(function (bundle) {
              return bundle.write({
                  // output format - 'amd', 'cjs', 'es6', 'iife', 'umd'
                  format: 'umd',
                  dest: path.join(dirs.build, files.main),
                  sourceMap: false,
                  banner: grunt.config('banner'),
                  intro: 'mx.version = \'' + grunt.config('pkg').version + '\';\n',
                  footer: '\n',
                  moduleName: 'mx'
              });
          });
      }
docs: transpile es6 modules