Fix the milc.cli.subcommand_name property value (#73)

The implementation of the `subcommand_name` property in `MILCInterface`
was wrong (it actually returned the `_subcommand` function object
instead of the subcommand name).

milc/milc_interface.py

@@ -84,8 +84,8 @@ def subcommands(self) -> Dict[str, Any]:
         return self.milc.subcommands
 
     @property
-    def subcommand_name(self) -> Any:
-        return self.milc._subcommand
+    def subcommand_name(self) -> Optional[str]:
+        return self.milc.subcommand_name
 
     def echo(self, text: str, *args: Any, **kwargs: Any) -> None:
         """Print colorized text to stdout.