Whoops, Redcarpet 2.x doesn't support footnotes.

Footnote support was completed in Redcarpet commit 40923d2ba687a611faec71cec02142cc84f08e67, which isn't included in the most recent 2.3.0 release or in the 2.2.x releases Jekyll is requiring. (Interestingly, it isn't in the 3.0.0 release either, although its history doesn't include the kill-off-1.8.7 commits that went into 3.0.0.

In order to enable footnotes, we'd have to:

- Ensure Redcarpet can be installed from git by Bundler. (I dunno if we've tried
  doing that with gems that build native extensions before.)
- Ensure Jekyll will work with Redcarpet 2.3.0 (currently, it pessimistically
  limits itself to 2.2.x).
- Ensure that none of the Redcarpet commits between 40923d and the point 2.3.0
  diverged from trunk will break 1.8.7 compatibility. As far as I can tell, the
  only ones that made real code changes were:
    - 9137e7cc - Ensure code spans put in emphasis work correctly
    - aead1ffb - Ensure nested parenthesis are handled into links
    - 855c0849 - Optional highlight support
    - 171904d5 - Fix FTP uris identified as emails
    - 16978cb4 - Remove the Sundown submodule from the repository
    - 20fbdc45 - Make emphasis wrapped inside parenthesis parsed
    - The run of commits between 2040782 and 40923d2, which constitute the
      footnote work proper.
- Update Jekyll to work with an unspecified version of Redcarpet (carrying
  patches again, huzzah).
- Pin Redcarpet to 40923d2 from git in our Gemfile.
- Add footnote support back to the _config.yml file.