Multi-line footnotes

Handles footnotes with hard-breaks in them according to PHP-Markdown's spec. Doesn't handle full multi-paragraph footnotes (#98).

Adds additional tests, and makes existing tests more closely match [PHP-Markdown's tests](https://github.com/michelf/mdtest/blob/master/PHP%20Markdown%20Extra.mdtest/Footnotes.text). There is still substantial work needed to bring this in line with PHP-Markdown.