T.P.App.CommandLineOptions: don't lowercase arg to --from/--read

This prevented users to use custom writers with uppercase characters in
their filenames. Format-normalization, including lower-casing of
format identifiers, happens during format parsing.

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -235,8 +235,7 @@ options :: [OptDescr (Opt -> ExceptT OptInfo IO Opt)]
 options =
     [ Option "fr" ["from","read"]
                  (ReqArg
-                  (\arg opt -> return opt { optFrom =
-                                              Just (T.toLower $ T.pack arg) })
+                  (\arg opt -> return opt { optFrom = Just $ T.pack arg })
                   "FORMAT")
                  ""