Consider migration to MMD6

[jasedit]

MultiMarkdown 6 offers faster parser with some amount of changes to parsing. Considering migrating to the new engine would potentially be useful.

[jasedit]

This is currently limited by fletcher/MultiMarkdown-6#38, which is required for many scriptorium documents using raw LaTeX source for fine-grained control of formatting issues. Other features have been issues as PR's or are awaiting in relevant branches.