exclude node_modules by default with register method

lib/6to5/node.js

@@ -5,8 +5,14 @@ var _         = require("lodash");
 exports.browserify = require("./browserify");
 exports.middleware = require("./middleware");
 
-exports.register = function () {
+exports.register = function (includeNodeModules) {
+  var old = require.extensions[".js"];
+
   require.extensions[".js"] = function (m, filename) {
+    if (!includeNodeModules && filename.indexOf("node_modules") >= 0) {
+      return old.apply(this, arugments);
+    }
+
     m._compile(exports.transformFileSync(filename, {
       sourceMap: true
     }), filename);