commonmark-hs style attributes?

The [commonmark-hs](https://github.com/jgm/commonmark-hs) library used by pandoc has a very nice [attributes extension](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/attributes.md) that allows attributes to be attached to any inline or block elements, e.g:

```
{.test}
* foo
* bar
```

attaches the attribute `class=test` to the list and `*test*{#ident}` results in `hi`.

How difficult would it be to implement the same for Marko?