Added the underscore node_module.

node_modules/underscore/modules/mixin.js

@@ -0,0 +1,18 @@
+import _ from './underscore.js';
+import each from './each.js';
+import functions from './functions.js';
+import { push } from './_setup.js';
+import chainResult from './_chainResult.js';
+
+// Add your own custom functions to the Underscore object.
+export default function mixin(obj) {
+  each(functions(obj), function(name) {
+    var func = _[name] = obj[name];
+    _.prototype[name] = function() {
+      var args = [this._wrapped];
+      push.apply(args, arguments);
+      return chainResult(this, func.apply(_, args));
+    };
+  });
+  return _;
+}