Markdown reader: don't parse footnote body unless extension enabled.

src/Text/Pandoc/Readers/Markdown.hs

@@ -460,8 +460,9 @@ rawLines = do
   return $ T.unlines (first:rest)
 
 noteBlock :: PandocMonad m => MarkdownParser m (F Blocks)
-noteBlock = try $ do
-  pos <- getPosition
+noteBlock = do
+  guardEnabled Ext_footnotes
+  do pos <- getPosition
      skipNonindentSpaces
      ref <- noteMarker
      char ':'
@@ -476,7 +477,8 @@ noteBlock = try $ do
      case M.lookup ref oldnotes of
        Just _  -> logMessage $ DuplicateNoteReference ref pos
        Nothing -> return ()
-  updateState $ \s -> s { stateNotes' = M.insert ref (pos, parsed) oldnotes }
+     updateState $ \s -> s { stateNotes' =
+       M.insert ref (pos, parsed) oldnotes }
      return mempty
 
 --