Add support for footnotes syntax in ghost. Compatible with showdown standalone

npm install showdown-ghost-footnotes

1. Require Showdown const showdown = require('showdown');
2. Require Showdown Footnotes const footnotes = require('showdown-ghost-footnotes');
3. Extend Showdown to enable Footnotes const converter = new showdown.Converter({ extensions: [footnotes] });

I have more [^1] to say up here.

Turns into

<p>
  I have more <sup id="fnref-1"><a href="#fn-1" rel="footnote" title="go to footnote">1</a></sup> to say up here.
</p>

And at the bottom of the page

[^1]: To say down here.

Turns into

<div class="footnotes">
<hr>
  <ol>
    <li id="fn-1"><p>To say down here.
    <a href="#fnref-1" class="footnote-backref" title="return to article">↩</a></p></li>
  </ol>
</div>