Markdown reader: Allow reference link labels starting with @...

...if citations extension disabled.  Example:  in

    [link text][@a]

    [@a]: url

`link text` isn't hyperlinked because `[@a]` is parsed as a citation.
Previously this happened whether or not the `citations` extension was
enabled. Now it happens only if the `citations` extension is enabled.

Closes #3209.

src/Text/Pandoc/Readers/Markdown.hs

@@ -1774,7 +1774,8 @@ referenceLink :: (Attr -> String -> String -> Inlines -> Inlines)
 referenceLink constructor (lab, raw) = do
   sp <- (True <$ lookAhead (char ' ')) <|> return False
   (_,raw') <- option (mempty, "") $
-      lookAhead (try (spnl >> normalCite >> return (mempty, "")))
+      lookAhead (try (guardEnabled Ext_citations >>
+                      spnl >> normalCite >> return (mempty, "")))
       <|>
       try (spnl >> reference)
   when (raw' == "") $ guardEnabled Ext_shortcut_reference_links