Configure footnote rendering.

- The config file can provide FootnoteAnchorPrefix, which will be used
  by blackfriday when rendering to HTML. A value of `q:` has the effect
  of making the anchor for a footnote `[^footie]` be `fn:q:footie`. The
  default is `""`.

- The config file can provide FootnoteReturnLinkContents, which will be
  used by blackfriday when rendering to HTML. A value of `^` has the
  effect of making the return link be `^` instead of `[return]`.