Added format selection dialogue for audio-only playlists

To avoid unwanted multiple audio tracks downloads (e.g. from Soundcloud playlists) a dialogue with format selection (mp3 audio vs. original audio format) will pop up, so user can cancel or go on with downloading.

virga.app (23 MB), this bundle includes ffmpeg and youtube-dl, just download and use.

virga barebone.app (202 KB), bundle is fully functional (and very small), but does not include youtube-dl or ffmpeg (assuming you have them installed in your system).

Both bundles features are identical.