The footnotes extension does not work when the superscript extension is enabled

[Garen Torikian]

As reported in gjtorikian/commonmarker#217. If there's a file called test.txt:

Gimme footnote[^1]
This is ^superscript^

[^1]: Here is the footnote

cargo run -- -e footnotes,superscript test.txt produces:

<p>Gimme footnote[^1]
This is <sup>superscript</sup></p>

whereas cargo run -- -e footnotes test.txt produces:

<p>Gimme footnote<sup class="footnote-ref"><a href="#fn1" id="fnref1">1</a></sup>
This is ^superscript^</p>
<section class="footnotes">
<ol>
<li id="fn1">
<p>Here is the footnote <a href="#fnref1" class="footnote-backref">↩</a></p>
</li>
</ol>
</section>

[Asherah Connor]

Duplicate of #106.