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.

hooks.yaml

@@ -31,6 +31,12 @@
     entry: check-json
     language: python
     files: \.json$
+-   id: pretty-format-json
+    name: Pretty format JSON
+    description: This hook sets a standard for formatting JSON files.
+    entry: pretty-format-json
+    language: python
+    files: \.json$
 -   id: check-merge-conflict
     name: Check for merge conflicts
     description: Check for files that contain merge conflict strings.