Footnotes (addresses #14)

This is my attempt at adding support for footnotes and inline footnotes in the style of Pandoc (requested in #14):

```
This is a footnote[^1]^[and this is an inline footnote]

[^1]: the footnote text.

    may be multiple paragraphs.
```

...should become...

```html
<p>This is a footnote<sup class="footnote-ref" id="fnref:1"><a rel="footnote" href="#fn:1">1</a></sup><sup class="footnote-ref" id="fnref:and-this-is-an-i"><a rel="footnote" href="#fn:and-this-is-an-i">2</a></sup></p>
<div class="footnotes">

<hr />

<ol>
<li id="fn:1"><p>the footnote text.</p>

<p>may be multiple paragraphs.</p>
</li>
<li id="fn:and-this-is-an-i">and this is an inline footnote</li>
</ol>
</div>
```

If anybody wants to add benchmarks and more tests that would be great.

#### Caveats:

- Since blackfriday does not produce an AST, it is not that simple to append a link back to the footnote reference in the article. As such, I've skipped that functionality for the time being. Note that it needs to place a link in a different place depending on the last element in the block. If the last element in the footnote is a paragraph, it appends it to the end of the paragraph, but if it's a recognised block element, it'll need to create a new paragraph. I'm sure there are ways to do this, but I can't think of anything elegant.
- The way footnote reference IDs are generated may not be safe with running the parser several times in the course of the same document.
- I don't know TeX, so I just put an empty skeleton implementation in the LaTeX renderer to satisfy the interface.

Otherwise, both regular (referenced) footnotes and inline footnotes are supported.