Add support for Gitalk

[Palmidence]

https://github.com/gitalk/gitalk - This would be useful to centralize questions on the docs (for clarifications, or other reason) and allow discussions on the modules themselves. This has many advantages over just leaving a message in #mapmaking, and hooks right in with GitHub. It is easily installable with the script and CSS import.

<link rel="stylesheet" href="//unpkg.com/gitalk/dist/gitalk.css">

<script src="//unpkg.com/docsify/lib/plugins/gitalk.min.js"></script>
<script src="//unpkg.com/gitalk/dist/gitalk.min.js"></script>
<script>
  const gitalk = new Gitalk({
    clientID: 'Github Application Client ID',
    clientSecret: 'Github Application Client Secret',
    repo: 'Github repo',
    owner: 'Github repo owner',
    admin: ['Github repo collaborators, only these guys can initialize github issues'],
    // facebook-like distraction free mode
    distractionFreeMode: false
  })
</script>