Support entry.toc for HTML as well

[fluffy-critter]

Expected Behavior

entry.toc could be extended to support HTML <h1> et al tags, rather than being limited to Markdown ### markup.

Possible Solution

Make TOC generation a post-filter for the HTML, rather than part of the markdown.HtmlRenderer header() handler.

This would also allow entry.toc to work on Markdown entries which use HTML header tags for whatever reason.

Steps to Reproduce (for bugs)

Context