Support for format_tags

[DieSache]

Hi!

Very happy about this project, it's very useful to me 🎉 . I see that there currently is only support for stream_tags and I would love to see support for something like the CLI equivalent of ffprobe <file> -show_entries format_tags.

The options for format tags vary by container. The supported tags can be found here

If anybody is interested in a temporary solution, this is (a bad) one:

let description = String::from_utf8(
    Command::new("ffprobe")
        .arg(<path>)
        .args(["-show_entries", "format_tags=description"])
        .args(["-of", "csv=p=0"])
        .output()?
        .stdout,
    )?;

[theduke]

Yeah, this is sensible.

Pretty busy right now. I appreciate a PR.