Added topics to Matrix exercise. Fixes #852

config.json

       "core": true,
       "difficulty": 4,
       "slug": "matrix",
-      "topics": null,
+      "topics": [
+	"loops",
+	"arrays",
+	"pattern_matching",
+	"integers"
+	],
       "unlocked_by": null,
       "uuid": "c1d4e0b4-6a0f-4be9-8222-345966621f53"
     },