Add new hook pretty-format-json

This new hook allows to standardize one's JSON files (sorted key/4
spaces indent).
By default it just fails if any file is not complying with the standard,
but you can also pass the arg `--autofix` and the hook will
pretty-format the file itself.

Good in use combined with the `check-json` hook.

testing/resources/not_pretty_formatted_json.json

@@ -0,0 +1,5 @@
+{
+    "foo": "bar",
+    "alist": [2, 34, 234],
+    "blah": null
+}

testing/resources/pretty_formatted_json.json

@@ -0,0 +1,9 @@
+{
+  "alist": [
+    2,
+    34,
+    234
+  ],
+  "blah": null,
+  "foo": "bar"
+}