Added support to download audio output only

Summary

First of all thanks for providing awesone python scripts.
Sometime we need to download youtube video as audio files only. I have added that support in youtubeDownloader.py. I also observed "UnicodeEncodeError" error for few videos. Added fix for that also.

Description

Added option to download only audio output of youtube video

The changes are as follows:

• Modified code to configure ydl_opts['format'] option to only audio or audio+video based on input provide
• Observed "UnicodeEncodeError: 'charmap' codec can't encode character '\uff5c' in position 86: character maps to <undefined>" error while downloading few of the videos. Added 'restrictfilenames': 'True', option to fix this. Refer - Critical: | in yt videos throws an error yt-dlp/yt-dlp#5819

Checks

Validated downloads with multiple videos.

in the repository

• Made no changes that degrades the functioning of the repository
• Gave each commit a better title (unlike updated README.md)

in the PR

• Followed the format of the pull_request_template
• Made the Pull Request in a small level (for the creator's wellfare)
• Tested the changes you made

Thank You,

Shailesh Vaidya