Make explicit config switch for Twitter DM link

layouts/_default/single.html

@@ -11,5 +11,7 @@
     </div>
     {{ .Content }}
   </article>
+  {{ if .Site.Params.ShowTwitterDMLink }}
   <p id=feedback>Your polite comments are welcome. <a href="https://twitter.com/messages/compose?recipient_id={{ .Site.Params.AuthorTwitterID }}&text={{ .Title }}:%20">Send me a private message on Twitter.</a></p>
+  {{ end }}
 {{ end }}