Improve demo with autorefresh and textarea autoresize

[chrisjsewell]

Originally posted by @chrisjsewell in executablebooks/markdown-it-docutils#19 (comment)

I'll take your word that it works and check it out when it's built on GH pages (would be great if there was a way to preview this documentation in the PR, like I do with https://docs.readthedocs.io/en/stable/pull-requests.html)

Should also upstream this to https://github.com/executablebooks/markdown-it-plugin-template