Merge pull request #882 from Nik-Mouz/master

Added topics to Matrix exercise. Fixes #852

config.json

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