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]`.

commands/hugo.go

@@ -121,6 +121,8 @@ func InitializeConfig() {
 	viper.SetDefault("PygmentsUseClasses", false)
 	viper.SetDefault("DisableLiveReload", false)
 	viper.SetDefault("PluralizeListTitles", true)
+	viper.SetDefault("FootnoteAnchorPrefix", "")
+	viper.SetDefault("FootnoteReturnLinkContents", "")
 
 	if hugoCmdV.PersistentFlags().Lookup("buildDrafts").Changed {
 		viper.Set("BuildDrafts", Draft)