Custom <pre> / <code> attributes

[naiyerasif]

Please consider a way to add custom attributes on pre / code elements. Some usecases:

• Keyboard navigation of codeblocks: To make the codeblocks accessible for keyboard users, tabindex = "0" should be set on pre element so that they can scroll them with keyboard. As a short-term goal, we can add tabindex = "0" by default. In long-term, we can still keep tabindex="0" as default but allow people to customize it. More details can be found here: https://accessibilityinsights.io/info-examples/web/scrollable-region-focusable/
• Adding [data-*] attributes for customizing the behavior of codeblocks, e.g., adding a "Copy to Clipboard" button for specific codeblocks, displaying codeblock title, language, etc.

[fengzilong]

I'm willing to work on a PR for this feature when I get off work