Add topics to Secret Handshake excercise

config.json

       "core": true,
       "difficulty": 3,
       "slug": "secret-handshake",
-      "topics": null,
+      "topics": [
+        "loops",
+	"transforming",
+        "conditionals",
+	"cryptography"
+      ],
       "unlocked_by": null,
       "uuid": "71c7c174-7e2c-43c2-bdd6-7515fcb12a91"
     },