add evancz/elm-playground to elm.json

Author: evancz

elm.json

@@ -17,7 +17,8 @@
             "elm/time": "1.0.0",
             "elm-explorations/linear-algebra": "1.0.3",
             "elm-explorations/markdown": "1.0.0",
-            "elm-explorations/webgl": "1.0.1"
+            "elm-explorations/webgl": "1.0.1",
+            "evancz/elm-playground": "1.0.0"
         },
         "indirect": {
             "elm/bytes": "1.0.8",