Support for Classes on All Tag Types

[aretrosen]

Can we add classes to tags other than Heading? I couldn't find a solution for this. It might be useful to do so in cases like adding dir=auto to the classes of Tag::Paragraph or Tag::List. I might be missing a better approach.

Currently, I am achieving this for Tag::Heading using the following code:

let parser = Parser::new_ext(markdown_input, options).map(|event| match event {
    Event::Start(mut tag) => {
        if let Tag::Heading { classes, .. } = &mut tag {
            classes.push("dir=auto".into());
        }
        Event::Start(tag)
    }
    _ => event,
});

Can we do this in other ways that I might not be aware of, which might be applicable to Tag::Paragraph and others too?

[systemsoverload]

I think their implementation goes a bit too far, but something like this would be ideal for all some elements. My use case (currently achieved by a pretty nasty custom iterator) is for adding data attributes like [foo](https://my-link){: data-baz='bar' }