markdown2dita: superscript link for footnotes doesn't appear

[amyhiggins]

When I add a footnote in markdown, I can see the generated footnote in the output, but no superscript number or link to the footnote. For example, this markdown:

Here's a simple footnote[^1].

[^1]: This is the first footnote.

results in (no superscript number or link in main text):

Here's a simple footnote.

---

1. This is the first footnote.