config.json: add two new keys to online_editor (#295)

- `online_editor.ace_editor_language`: the language identifier for the
   Ace editor, which is used to edit code on the website.
- `online_editor.highlightjs_language`: the language identifier for
  Highlight.js, which is used to highlight code on the website.

config.json

@@ -12,7 +12,9 @@
   "version": 3,
   "online_editor": {
     "indent_style": "space",
-    "indent_size": 2
+    "indent_size": 2,
+    "ace_editor_language": "nim",
+    "highlightjs_language": "nim"
   },
   "files": {
     "solution": [