Don't let autolinks create reference links.

Previously we got bad results with -t markdown --reference-links
if the input contained autolinks like <http://yahoo.com>.

src/Text/Pandoc/Writers/Markdown.hs

@@ -498,10 +498,10 @@ inlineToMarkdown opts (Link txt (src, tit)) = do
                      then empty
                      else text $ " \"" ++ tit ++ "\""
   let srcSuffix = if isPrefixOf "mailto:" src then drop 7 src else src
-  let useRefLinks = writerReferenceLinks opts
   let useAuto = case (tit,txt) of
                       ("", [Code _ s]) | s == srcSuffix -> True
                       _                                 -> False
+  let useRefLinks = writerReferenceLinks opts && not useAuto
   ref <- if useRefLinks then getReference txt (src, tit) else return []
   reftext <- inlineListToMarkdown opts ref
   return $ if useAuto