Markdown to LaTeX: Level 6 Heading

As far as I understand Markdown defines headings upto 6 levels, to correspond to h1-h6 in HTML.

When Pandoc is converting a level 5 heading it is converted to \subparagraph{}, while a level 6 heading is just converted to plain text.

I think it would be better to have a level 6 heading wrapped in a \subsubparagraph{} macro, while having something like \newcommand\subsubparagraph[1]{#1} in default.latex. That way it would preserve the previous behavior, while also making it possible to write templates that modify the behavior of level 6 headings. This would help people like me who are writing templates for converting markdown to pdf.

I don't know haskell yet, so I can't really submit a pull request.