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.

pre_commit_hooks/pretty_format_json.py

@@ -0,0 +1,70 @@
+from __future__ import print_function
+
+import argparse
+import sys
+
+import simplejson
+
+
+def _get_pretty_format(contents, indent):
+    return simplejson.dumps(
+        simplejson.loads(contents),
+        sort_keys=True,
+        indent=indent
+    ) + "\n"  # dumps don't end with a newline
+
+
+def _autofix(filename, new_contents):
+    print("Fixing file {0}".format(filename))
+    with open(filename, 'w') as f:
+        f.write(new_contents)
+
+
+def pretty_format_json(argv=None):
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        '--autofix',
+        action='store_true',
+        dest='autofix',
+        help='Automatically fixes encountered not-pretty-formatted files'
+    )
+    parser.add_argument(
+        '--indent',
+        type=int,
+        default=2,
+        help='Number of indent spaces used to pretty-format files'
+    )
+
+    parser.add_argument('filenames', nargs='*', help='Filenames to fix')
+    args = parser.parse_args(argv)
+
+    status = 0
+
+    for json_file in args.filenames:
+        try:
+            f = open(json_file, 'r')
+            contents = f.read()
+            f.close()
+
+            pretty_contents = _get_pretty_format(contents, args.indent)
+
+            if contents != pretty_contents:
+                print("File {0} is not pretty-formatted".format(json_file))
+
+                if args.autofix:
+                    _autofix(json_file, pretty_contents)
+
+                status = 1
+
+        except simplejson.JSONDecodeError:
+            print(
+                "Input File {0} is not a valid JSON, consider using check-json"
+                .format(json_file)
+            )
+            return 1
+
+    return status
+
+
+if __name__ == '__main__':
+    sys.exit(pretty_format_json())