Add Isogram topics

referencing #788

conditionals: finding if a char is repeated
loops: looping through a string's characters
parsing: parsing a string for repeated characters
strings: self-explanatory :)

config.json

       "core": false,
       "difficulty": 4,
       "slug": "isogram",
-      "topics": null,
+      "topics": [
+        "conditionals",
+        "loops",
+        "strings",
+        "algorithms",
+        "parsing",
+      ],
       "unlocked_by": "hello-world",
       "uuid": "c3e89c7c-3a8a-4ddc-b653-9b0ff9e1d7d8"
     },