[Feature Request] Arbitrary code-fence attributes with data-* prefix

[colelawrence]

I'd like to be able to add arbitrary information on different code blocks by being able to do something like

```rs,data-error="true"
fn wrong_syntax<'a>() {
}
```

This would make for an easy escape hatch to add arbitrary functionality to different code blocks.

[Keats]

Makes sense, but only once there will be a new highlighter, no point changing it currently