Feature footnotes

Footnotes feature. It looks like MultiMarkdown Footnotes Syntax:

```
Here is some text containing a footnote.[^somesamplefootnote]

[^somesamplefootnote]: Here is the text of the footnote itself.
```

- http://daringfireball.net/2005/07/footnotes
- https://github.com/fletcher/MultiMarkdown/wiki/MultiMarkdown-Syntax-Guide#wiki-footnotes
- http://six.pairlist.net/pipermail/markdown-discuss/2005-August/001442.html
- http://six.pairlist.net/pipermail/markdown-discuss/2005-August/001480.html

Related Issue: https://github.com/chjj/marked/issues/27

@chjj Have a preview of this patch. I will add test cases soon.