Feature request: Markdown for option labels

Similarly to how labels can render markdown with the `markdown` option #227, perhaps an option like `markdown_options` for `checkboxes` and `radio`?

Happy to submit the PR.

Start by reviewing the markdown option described in issue #227, then trace how checkbox and radio labels are rendered and where their options are handled. Confirm the intended behavior for a markdown_options setting and identify the relevant tests or examples; the work is done when both field types render option labels consistently with the existing markdown behavior.

Written by the indexing model from the issue text.