Add markdown-it-gfm package

[Chris Sewell]

As a meta package to enable all CommonMark, plus all GFM extensions: https://github.github.com/gfm/

• Extension not yet created: https://github.github.com/gfm/#disallowed-raw-html-extension-
• Should heading anchors be included (optional feature?)

[Chris Sewell]

added in #19