First support for md extensions; Package all needed dependencies in the repo

Source/node_modules/core-js/modules/es6.object.keys.js

@@ -0,0 +1,9 @@
+// 19.1.2.14 Object.keys(O)
+var toObject = require('./_to-object');
+var $keys = require('./_object-keys');
+
+require('./_object-sap')('keys', function () {
+  return function keys(it) {
+    return $keys(toObject(it));
+  };
+});