Implement video message configuration

* Config is update when new key is added (always fully explicit)

breadtube_bot/config.py

@@ -14,6 +14,7 @@ class Config:
     unmanaged_categories: str = ''
     youtube_channel_refresh_interval: float = 3600
     youtube_channel_video_count: int = 10
+    youtube_channel_video_message: str = '[{{video_title}}](https://www.youtube.com/video/{{video_id}})'
 
     def to_str(self) -> str:
         return '\n'.join(['config', *[f'{k}={v}' for k, v in asdict(self).items()]])