Improve video_id extraction in YoutubeLoader (#4452)
# Improve video_id extraction in `YoutubeLoader`
`YoutubeLoader.from_youtube_url` can only deal with one specific url
format. I've introduced `YoutubeLoader.extract_video_id` which can
extract video id from common YT urls.
Fixes #4451
@eyurtsev
---------
Co-authored-by: Kamil Niski <kamil.niski@gmail.com>
