Consider switching to GitHub-flavoured Markdown parser in Kramdown

[floehopper]

Kramdown supports an non-default parser for GitHub-flavoured Markdown. I think this might be a sensible one to use. I found myself wanting it just now when I wanted to be able to use the "strikethrough" functionality.

It looks as if this is as simple as changing:

Kramdown::Document.new(content).to_html

to:

Kramdown::Document.new(content, input: 'GFM').to_html

However, that change seems to affect the IDs automatically added to headings, so we'd need to fix any links to those anchors.