fix crash when --prompt="prompt" is used in CLI

- The following were supposed to be equivalent, but the latter crashes:
```
invoke> banana sushi
invoke> --prompt="banana sushi"
```
This PR fixes the problem.

- Fixes #2548

ldm/invoke/args.py

@@ -230,7 +230,8 @@ class Args(object):
                     switches = ''
         try:
             self._cmd_switches = self._cmd_parser.parse_args(shlex.split(switches,comments=True))
-            setattr(self._cmd_switches,'prompt',prompt)
+            if not getattr(self._cmd_switches,'prompt'):
+                setattr(self._cmd_switches,'prompt',prompt)
             return self._cmd_switches
         except:
             return None