Added the underscore node_module.

node_modules/underscore/modules/once.js

@@ -0,0 +1,6 @@
+import partial from './partial.js';
+import before from './before.js';
+
+// Returns a function that will be executed at most one time, no matter how
+// often you call it. Useful for lazy initialization.
+export default partial(before, 2);