Video support via youtube-dl

[krzygorz]

This adds basic support for video posts via youtube-dl with a fallback to an alternative extractor for low-quality mobile versions of the videos.

Some notes:

• This asks youtube-dl for a single link of highest possible quality. I think it's possible to get a slightly better result by merging separate video and audio files but then we'd have to download the files instead of just returning an url.
• youtube-dl is added to pyproject.toml as an optional dependency
• youtube-dl is turned on by passing youtube_dl=True to get_posts, I'm not sure if this is an optimal choice.

[kevinzg]

Awesome, thanks for your contribution!