Add: the-economist gre (#2043)

* Add /the-economist/gre-vocabulary

* 原来有接口 = =

* 忘了写文档 = =

lib/router.js

@@ -1306,4 +1306,7 @@ router.get('/saraba1st/thread/:tid', require('./routes/saraba1st/thread'));
 router.get('/gradcafe/result/:type', require('./routes/gradcafe/result'));
 router.get('/gradcafe/result', require('./routes/gradcafe/result'));
 
+// The Economist
+router.get('/the-economist/gre-vocabulary', require('./routes/the-economist/gre-vocabulary'));
+
 module.exports = router;