mdwn: Don't enable alphabetically labelled ordered lists by default

mdwn: Don't enable alphabetically labelled ordered lists by default

This avoids misinterpreting initials ("C. S. Lewis was an author"),
the abbreviation for Monsieur ("M. Descartes was a philosopher") and
German page numbering ("S. 42") as ordered lists if they happen to
begin a line.

This only affects the default Discount implementation: Text::Markdown
and Text::MultiMarkdown do not have this feature anyway. A new
mdwn_alpha_list option can be used to restore the old interpretation.