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]`.

content/overview/configuration.md

@@ -40,6 +40,7 @@ The following is an example of a toml config file with some of the default value
     builddrafts = false
     baseurl = "http://yoursite.example.com/"
     canonifyurls = true
+
     [indexes]
        category = "categories"
        tag = "tags"
@@ -49,6 +50,7 @@ Here is a yaml configuration file which sets a few more options
     ---
     baseurl: "http://yoursite.example.com/"
     title: "Yoyodyne Widget Blogging"
+    footnotereturnlinkcontents: "↩"
     permalinks:
       post: /:year/:month/:title/
     params: