Add --extend-select command line argument

Implement `--extend-select` command line argument following what was
done for `--extend-ignore` in !233. This option can be used to
selectively add individual codes without overriding the default list
entirely.

Addresses the remaining item of issue #1061.

tests/unit/test_style_guide.py

@@ -14,6 +14,7 @@ def create_options(**kwargs):
     """Create and return an instance of argparse.Namespace."""
     kwargs.setdefault('select', [])
     kwargs.setdefault('extended_default_select', [])
+    kwargs.setdefault('extend_select', [])
     kwargs.setdefault('ignore', [])
     kwargs.setdefault('extend_ignore', [])
     kwargs.setdefault('disable_noqa', False)