Added "python -m configurations" entry point.

inspired by https://github.com/django/django/blob/e0a46367df8b17905f1c78f5c86f88d21c0f2b4d/django/__main__.py#L1-L9

see also https://code.djangoproject.com/ticket/24857

configurations/__main__.py

@@ -0,0 +1,10 @@
+"""
+invokes django-cadmin when the configurations module is run as a script.
+
+Example: python -m configurations check
+"""
+
+from .management import execute_from_command_line
+
+if __name__ == "__main__":
+    execute_from_command_line()

tests/test_main.py

@@ -106,6 +106,22 @@ def test_configuration_argument_in_cli(self):
                                 stdout=subprocess.PIPE)
         self.assertIn('--configuration', proc.communicate()[0].decode('utf-8'))
 
+    def test_configuration_argument_in_runypy_cli(self):
+        """
+        Verify that's configuration option has been added to managements
+        commands when using the -m entry point
+        """
+        proc = subprocess.Popen(
+            [sys.executable, '-m', 'configurations', 'test', '--help'],
+            stdout=subprocess.PIPE,
+        )
+        self.assertIn('--configuration', proc.communicate()[0].decode('utf-8'))
+        proc = subprocess.Popen(
+            [sys.executable, '-m', 'configurations', 'runserver', '--help'],
+            stdout=subprocess.PIPE,
+        )
+        self.assertIn('--configuration', proc.communicate()[0].decode('utf-8'))
+
     def test_django_setup_only_called_once(self):
         proc = subprocess.Popen(
             [sys.executable, os.path.join(os.path.dirname(__file__),