Add support for table of contents rendering

[fluffy-critter]

Expected Behavior

There should be some means of signaling that we should render a table of contents in the more-text.

With this mechanism, we would set nesting_level on the HtmlRenderer and also run an HtmlTocRenderer with the same nesting level.

Possible Solution

entry._get_markup() could also take a show_toc=N parameter, which is set to None on entry.text and to a value indicated by a Toc-Level header on the entry itself. Perhaps it should take the minimum of the entry's value and the template's value (so both the template and the entry need to want a TOC for it to display).

The TOC content itself could be wrapped in a <div class="toc"> (or maybe an id with the caveat that it only makes sense to render a single entry's more content on a single page).

Context

Would be great for the manual, among other things.

[fluffy-critter]

In the interest of generality I'd actually like to see the TOC renderer be called explicitly, like:

{% if entry.toc %}
<div class="toc">{{entry.toc}}</div>
{% endif %}

where entry.toc is truthy if Toc-Level is set, and renders out the HtmlTocRenderer for entry.more. And then entry.more would always use Toc-Level for its nesting_level parameter.

It's also tempting to just have the template itself indicate whether it should use HtmlTocRenderer or not, and set the nesting_level, so that entry.text can get it too, but I'm fine with limiting TOCs to entry.more.