Allow MarkupFormatter.Options.OrderedListNumerals to defer to the Markdown for list start index

[QuietMisdreavus]

Radar link: rdar://99970544

Right now, the OrderedListNumerals set of options requires you to provide the list's starting index yourself. Once #22 is merged, it can read that directly from the Markdown. We should provide the option to the formatter to defer to the list's starting index, likely by providing nil to either of these integer parameters.