Render markdown (including bill of materials)

[openvmp]

There is currently dead code "Shape.render_markdown()" copied from OpenVMP's internal library.
It has to move to the "Assembly" class.
The "Part" class (the other child of "Shape") needs to get another implementation of "render_markdown()" that creates a simple description of the part.

Extend "Project.render()" to render markdown files if "render->markdown" is provided.
Render markdown for assemblies unless "render->markdown->assemblies==False".
Render markdown for parts if "render->markdown->parts==True".
Pass all other options necessary to render markdown files as other parameters in "render->markdown".

[clairbee]

The only thing left is rendering the bill of materials for assemblies.