Added the underscore node_module.

node_modules/underscore/modules/countBy.js

@@ -0,0 +1,9 @@
+import group from './_group.js';
+import has from './_has.js';
+
+// Counts instances of an object that group by a certain criterion. Pass
+// either a string attribute to count by, or a function that returns the
+// criterion.
+export default group(function(result, value, key) {
+  if (has(result, key)) result[key]++; else result[key] = 1;
+});