Configuration toggle to add parsed line number to elements as data-line attribute

**Describe the feature** Add configuration option (defaulting to false/off) that when true/on adds a custom user attribute to generated elements to indicate the line number of the parsed output came from. **Why is this feature necessary?** We've had requests of users in our tool to be able to select a bit of the HTML output and use that to find the corresponding source markdown. This would also aid in our efforts for auto-pagination. **Describe alternatives you've considered** Manually walking the DOM results and attempting to match the text contents of elemnts to strings in the source text.