Show unresolved references using eprintf not log-warning
Rename `other` to `other-block`
Add support for footnotes. Closes #1.

Follows the Python-Markdown description:

http://pythonhosted.org/Markdown/extensions/footnotes.html

- Footnotes labels may be any text, but the displayed footnotes are
  automatically numbered.

- Footnote definitions may be a full block of markdown (e.g. contain
  multiple paragraphs, block quotes, code blocks, etc.).

- "Back links" (i.e.  ) are provided to jump from the definition back
  to the spot in the main text.

This commit also deletes an extraneous HTML attribute we were
generated for anchor tags for headings.