Footnotes implementation

[panrus]

To create a footnote reference, add a caret and an identifier inside brackets ([^1]).
Add the footnote using another caret and number inside brackets with a colon and text ([^1]: My footnote.). You don’t have to put footnotes at the end of the document. You can put them anywhere.

Here is an example:

The implementation focus is to be fast (see benchmark) and extensible [^1]

[^1]: See [benchmark](https://github.com/kzykhys/Markbench#readme).

[samwilson]

It'd be great to have footnote support!

There's a typo in the test filenames by the way, which is why the tests are passing.

Note that there's a MarkdownExtra test case here:
https://github.com/michelf/php-markdown/blob/lib/test/resources/php-markdown-extra.mdtest/Footnotes.text
Maybe it could be used verbatim, to ensure compatibility with that project?