Add --no-mux command-line option to allow downloading individual track
files without muxing them into a container file (.mkv/.mka/.mks).
This addresses use cases where users want to download tracks separately,
such as:
- Downloading only subtitles as individual .srt/.vtt files
- Keeping audio/video/subtitle tracks as separate files
- Converting subtitle formats without creating container files
When --no-mux is used:
- Tracks are saved as individual files with descriptive suffixes
- Video tracks: filename.{codec}.ext
- Audio tracks: filename.{language}.{codec}.ext
- Subtitle tracks: filename.{language}.forced.sdh.ext (as applicable)
- Folder structure respects --no-folder flag
Resolves#21
d6f8e42f52