[O] Customizing TOCs in Derived Exporters

[Josiah Schwab]

Hi All,

I'm working on writing a derived exporter and I've been using ox-md.el
as a template/guide.

If I use the markdown exporter to export the following org file

* Meeting Notes
** First Topic
*** Subtopic 1
+ a point
+ an *important* point
*** Subtopic 2
+ another point
+ an /interesting/ point
** Second Topic
One long announcement without any particular structure.

an html table of contents appears at the top of the exported file.

<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#sec-1">1. Meeting Notes</a>
<ul>
<li><a href="#sec-1-1">1.1. First Topic</a>
<ul>
<li><a href="#sec-1-1-1">1.1.1. Subtopic 1</a></li>
<li><a href="#sec-1-1-2">1.1.2. Subtopic 2</a></li>
</ul>
</li>
<li><a href="#sec-1-2">1.2. Second Topic</a></li>
</ul>
</li>
</ul>
</div>
</div>
# Meeting Notes

## First Topic

### Subtopic 1

-   a point

-   an **important** point

### Subtopic 2

-   another point

-   an *interesting* point

## Second Topic

One long announcement without any particular structure.

How does one go about overriding org-html-toc, which unlike something
like org-html-bold, does not appear in org-export-define-backend?  I
don't want the TOC not to appear, rather want it to be generated by
another function, org-md-toc or what have you.

Thanks,
Josiah