[media.vlc] Always add --play-and-exit to args.

Without this option VLC seems to hang in the background, and it may
cause PulseAudio failures or memory issues after several videos have
been played - each in its own session.

platypush/plugins/media/vlc/__init__.py

@@ -42,7 +42,10 @@ class MediaVlcPlugin(MediaPlugin):
 
         super().__init__(**kwargs)
 
-        self._args = args or []
+        self._args = list(args or [])
+        if '--play-and-exit' not in self._args:
+            self._args.append('--play-and-exit')
+
         self._instance = None
         self._player = None
         self._latest_seek = None