Allow Jekyll posts to render a table of contents

[0xdevalias]

This doesn't appear to be natively supported in commonmark, nor in any Github Flavoured Markdown (GFM) extensions i've seen:

• HTML table-of-contents renderer commonmark/cmark#78

It seems there are a number of Jekyll plugins that would allow us to do this however, eg:

• https://github.com/toshimaru/jekyll-toc
• https://github.com/allejo/jekyll-toc