Markdown reader: Be less restrictive about class attributes.

This allows attributes like `C++`.  Any nonspace and non-}.

src/Text/Pandoc/Readers/Markdown.hs

@@ -506,7 +506,7 @@ identifierAttr = try $ do
 classAttr :: Parser [Char] st (String, [String], [a])
 classAttr = try $ do
   char '.'
-  result <- identifier
+  result <- many1 $ noneOf " \t\n\r}"
   return ("",[result],[])
 
 keyValAttr :: Parser [Char] st (String, [a], [(String, String)])