Markdown reader: Handle c++ and objective-c as language identifiers

in github-style fenced blocks.  Closes #1318.

Note:  This is special-case handling of these two cases.
It would be good to do something more systematic.

src/Text/Pandoc/Readers/Markdown.hs

@@ -618,12 +618,19 @@ codeBlockFenced = try $ do
   skipMany spaceChar
   attr <- option ([],[],[]) $
             try (guardEnabled Ext_fenced_code_attributes >> attributes)
-           <|> ((\x -> ("",[x],[])) <$> identifier)
+           <|> ((\x -> ("",[toLanguageId x],[])) <$> many1 nonspaceChar)
   blankline
   contents <- manyTill anyLine (blockDelimiter (== c) (Just size))
   blanklines
   return $ return $ B.codeBlockWith attr $ intercalate "\n" contents
 
+-- correctly handle github language identifiers
+toLanguageId :: String -> String
+toLanguageId = map toLower . go
+  where go "c++" = "cpp"
+        go "objective-c" = "objectivec"
+        go x = x
+
 codeBlockIndented :: MarkdownParser (F Blocks)
 codeBlockIndented = do
   contents <- many1 (indentedLine <|>