Enhancement - adding audio only downloads.

[NicholasDewberryOfficial]

As stated in #9 a lot of us use youtube to listen to albums.

I modified modes.go in two significant ways:

1. I added an "Audio" mode (case sensitive) that users can select if they only want to hear audio.
2. I added an override that changes the yt-dlp settings if the "Audio" mode is activated (done by hard checking the format string).

The override is a little clunky, but I did it this way because it might be good to have a long-term division of args for video/audio and args only for audio.

I can change my implementation if it's not sufficient. With that being said - it works on my machine, and video works too.

The only problem I have is that I can't stream videos in audio only mode. Streaming always defaults to high quality video.

[KrishnaSSH]

thanks for the PR ❤️