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.

tests/pretty_format_json_test.py

@@ -0,0 +1,40 @@
+import pytest
+import tempfile
+
+from pre_commit_hooks.pretty_format_json import pretty_format_json
+from testing.util import get_resource_path
+
+
+@pytest.mark.parametrize(('filename', 'expected_retval'), (
+    ('not_pretty_formatted_json.json', 1),
+    ('pretty_formatted_json.json', 0),
+))
+def test_pretty_format_json(filename, expected_retval):
+    ret = pretty_format_json([get_resource_path(filename)])
+    assert ret == expected_retval
+
+
+def test_autofix_pretty_format_json():
+    toformat_file = tempfile.NamedTemporaryFile(delete=False, mode='w+')
+
+    # copy our file to format there
+    model_file = open(get_resource_path('not_pretty_formatted_json.json'), 'r')
+    model_contents = model_file.read()
+    model_file.close()
+
+    toformat_file.write(model_contents)
+    toformat_file.close()
+
+    # now launch the autofix on that file
+    ret = pretty_format_json(['--autofix', toformat_file.name])
+    # it should have formatted it
+    assert ret == 1
+
+    # file already good
+    ret = pretty_format_json([toformat_file.name])
+    assert ret == 0
+
+
+def test_badfile_pretty_format_json():
+    ret = pretty_format_json([get_resource_path('ok_yaml.yaml')])
+    assert ret == 1