Add default download mode and max video quality settings to configuration file

yt-playlist-config.json

@@ -9,6 +9,8 @@
   "yt_dlp_path": "./bin/yt-dlp.exe",
   "ffmpeg_path": "./bin/ffmpeg.exe",
   "aria2c_path": "./bin/aria2c.exe",
+  "download_mode": "audio",  
+  "max_video_quality": "1080p",
   "max_parallel_downloads": 10,
   "aria2c_connections": 8
 }