Add a way for the TOC to use draw non-header IDs

[briandominick]

It would be handy to have objects in a reference load in the page-nav's TOC, even if they don't have their own headers, which most will not.

The solution here could possibly include adding hidden headers, though that's not perfectly elegant.

What I'm thinking is if you have a DL reference, for instance...

[#setting-a.toc-add]
setting-a:: Here is the definition of setting-a.

[#setting-b.toc-add]
setting-b:: Here is the definition of setting-b.