add two-fer as unlock exercise for reverse-string

config.json

@@ -30,7 +30,7 @@
       "topics": [
         "strings"
       ],
-      "unlocked_by": null,
+      "unlocked_by": "two-fer",
       "uuid": "2c8afeed-480e-41f3-ad58-590fa8f88029"
     },
     {