Generate TOC from headings

[thephilgray]

Incorporate https://github.com/thephilgray/node-toc-generator to parse headings from markup and pass toc object to templates

[thephilgray]

This has been implemented in local branches, but it breaks another feature.

Currently, the TOC is built from metadata.yaml. This is fine. Ideally, the autogenerated TOC should be implemented as an option. It's possible to just create separate TOC templates for autogenerated and manual. But this is creates poor development experience, not to mention that if you choose not to use the autogenerated template, you will still pay the cost in computing.

For better development experience:

• It should be a separate task that can be turned on or off
• It should be implemented in a way that allows you to use metadata.yaml to refine the autogenerated TOC data (via merge)