Extend Markdown output to include headers & arbitrary text

[NicMcPhee]

After adding basic support for generating Markdown (#69), it would be good to extend that to create "stand-alone" documents with headers/titles, and any other arbitrary text that the user might wish to include before and/or after the proof.

This might be overkill, as it's easy enough for them to add such text in a Markdown editor after they've downloaded the proof. But it might be nice to have a "whole package" option for those who don't want to figure out local editing, etc.