Added the underscore node_module.

node_modules/underscore/modules/flatten.js

@@ -0,0 +1,7 @@
+import _flatten from './_flatten.js';
+
+// Flatten out an array, either recursively (by default), or up to `depth`.
+// Passing `true` or `false` as `depth` means `1` or `Infinity`, respectively.
+export default function flatten(array, depth) {
+  return _flatten(array, depth, false);
+}