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]`.