RST writer: Use .. code:: language for code blocks with language.

Closes #721.
Also fixed whitespace in lhs tests.

src/Text/Pandoc/Writers/RST.hs

@@ -177,7 +177,12 @@ blockToRST (CodeBlock (_,classes,_) str) = do
   if "haskell" `elem` classes && "literate" `elem` classes &&
                   isEnabled Ext_literate_haskell opts
      then return $ prefixed "> " (text str) $$ blankline
-     else return $ "::" $+$ nest tabstop (text str) $$ blankline
+     else return $
+          (case [c | c <- classes,
+                     c `notElem` ["sourceCode","literate","numberLines"]] of
+             []       -> "::"
+             (lang:_) -> ".. code:: " <> text lang)
+          $+$ nest tabstop (text str) $$ blankline
 blockToRST (BlockQuote blocks) = do
   tabstop <- get >>= (return . writerTabStop . stOptions)
   contents <- blockListToRST blocks

tests/lhs-test.rst

@@ -4,7 +4,7 @@ lhs test
 ``unsplit`` is an arrow that takes a pair of values and combines them to
 return a single value:
 
-::
+.. code:: haskell
 
     unsplit :: (Arrow a) => (b -> c -> d) -> a (b, c) d
     unsplit = arr . uncurry