passage: add footnote parsing to passage-markdown

Summary:
```
Adds basic footnote syntax, which takes any `^` characters
in a passage and places a superscript denoting a footnote
there, which content creators can put in the "footnotes"
box of the widget.

Some other possibilities we could implement are having
inline footnotes  like^(this), or reference footnotes
that reference a footnote declared later^[1]

[1]: such as down here
```

Test Plan: `make test`

Reviewers: michelle

Reviewed By: michelle

Subscribers: emily

Differential Revision: http://phabricator.khanacademy.org/D12715