add: option to return non-zero if files are changed

Author: chadrik

src/mdformat/_cli.py

@@ -178,6 +178,8 @@ def run(cli_args: Sequence[str], cache_toml: bool = True) -> int:  # noqa: C901
                 return 1
             if path:
                 if formatted_str != original_str:
+                    if opts["exit_non_zero_on_format"]:
+                        format_errors_found = True
                     path.write_bytes(formatted_str.encode())
             else:
                 sys.stdout.buffer.write(formatted_str.encode())
@@ -210,6 +212,11 @@ def make_arg_parser(
     parser.add_argument(
         "--check", action="store_true", help="do not apply changes to files"
     )
+    parser.add_argument(
+        "--exit-non-zero-on-format", 
+        action="store_true", 
+        help="return non-zero if files are modified",
+    )
     parser.add_argument(
         "--no-validate",
         action="store_const",