Add TOC support for Markdown output

[jautor]

Previously Markdown output was targeted for Slate online documentation and therefore did not need a Table of Contents. But general Markdown output (non-Slate) needs this function enabled.

[jautor]

This may not be necessary as we have support for ToC generation in the publication process. But it may be useful for others that wish to use the markdown output.

[akf]

Done!