Footnotes extension for Showdown

See #1318

This adds the footnote syntax from [Markdown Extra](https://michelf.ca/projects/php-markdown/extra/):

```
That's some text with a footnote.[^1]

[^1]: And that's the footnote.
```

It outputs HTML matching the style of [bigfoot.js](http://www.bigfootjs.com/) (but bigfoot.js is not required):

```
<p>That's some text with a footnote.<sup id="fnref:1"><a>1</a></sup></p>

<div class="footnotes"><ol><li class="footnote" id="fn:1"><p>And that's the footnote. <a title="return to article">↩</a></p></li></ol></div>
```

For convenience, I also included [^n] syntax which automatically numbers them by their appearance in the document.

Example usage in production: https://mispy.me/2014/10/extending-the-markdown-syntax-in-ghost/