Make sure all anchors are created for RST links

[felixfontein]

Fixes #1535.

Running the script on the source of https://gist.github.com/felixfontein/5ec5ad555ba6cdb293beb04c83280c90 produces:

<a name="test-section"></a>
<h2>Test section</h2>
<p>Link: <a class="reference internal" href="#target">link to target</a>.</p>
<a name="target-section"></a>
<a name="target"></a>
<h2>Target section</h2>
<p>The link should point to this section's title.</p>

Here both expected anchors (the auto-generated one from the section title, and the one from the label) are created.

[felixfontein]

Is there actually any interest in contributions to this repository? If not, maybe that should be explicitly mentioned... https://github.com/github/markup/blob/master/CONTRIBUTING.md seemed to indicate to me that contributions are wanted...