Add mutmut --version flag

Author: Otto-AA

mutmut/__main__.py

@@ -690,6 +690,7 @@ def read_config():
 
 
 @click.group()
+@click.version_option()
 def cli():
     pass
 

tests/e2e/test_cli_version.py

@@ -0,0 +1,9 @@
+from mutmut import __version__
+from mutmut.__main__ import cli
+from click.testing import CliRunner
+
+def test_cli_version():
+    result = CliRunner().invoke(cli, ["--version"])
+
+    assert result.exit_code == 0
+    assert __version__ in result.output