Write id for code block to label attr in latex when listing is used

The code:

    ~~~{#test}
    asdf
    ~~~

gets compiled to html:

    <pre id="test">
    asdf
    </pre>

So it is possible to link to the identifier `test`

But this doesn't happen on latex

When using the listings package (`--listings`) it is possible to set the
identifier using the `label=test` property:

    \begin{lstlisting}[label=id]
    hi
    \end{lstlisting}

And this is exactly what this patch is doing.

Modified LaTeX Reader/Writer and added tests for this.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -47,6 +47,7 @@ import Text.Pandoc.Builder
 import Data.Char (isLetter)
 import Control.Applicative
 import Data.Monoid
+import Data.Maybe (fromMaybe)
 import System.Environment (getEnv)
 import System.FilePath (replaceExtension, (</>))
 import Data.List (intercalate, intersperse)
@@ -901,7 +902,7 @@ environments = M.fromList
                                       lookup "numbers" options == Just "left" ]
                                  ++ maybe [] (:[]) (lookup "language" options
                                          >>= fromListingsLanguage)
-                      let attr = ("",classes,kvs)
+                      let attr = (fromMaybe "" (lookup "label" options),classes,kvs)
                       codeBlockWith attr <$> (verbEnv "lstlisting"))
   , ("minted",     do options <- option [] keyvals
                       lang <- grouped (many1 $ satisfy (/='}'))

src/Text/Pandoc/Writers/LaTeX.hs

@@ -313,7 +313,7 @@ blockToLaTeX (BlockQuote lst) = do
        _ -> do
          contents <- blockListToLaTeX lst
          return $ "\\begin{quote}" $$ contents $$ "\\end{quote}"
-blockToLaTeX (CodeBlock (_,classes,keyvalAttr) str) = do
+blockToLaTeX (CodeBlock (identifier,classes,keyvalAttr) str) = do
   opts <- gets stOptions
   case () of
      _ | isEnabled Ext_literate_haskell opts && "haskell" `elem` classes &&
@@ -344,7 +344,11 @@ blockToLaTeX (CodeBlock (_,classes,keyvalAttr) str) = do
                              [ (if key == "startFrom"
                                    then "firstnumber"
                                    else key) ++ "=" ++ attr |
-                                   (key,attr) <- keyvalAttr ]
+                                   (key,attr) <- keyvalAttr ] ++
+                             (if identifier == ""
+                                   then []
+                                   else [ "label=" ++ identifier ])
+
                         else []
                printParams
                    | null params = empty

tests/Tests/Readers/LaTeX.hs

@@ -55,6 +55,13 @@ tests = [ testGroup "basic"
             "hi % this is a comment\nthere\n" =?> para "hi there"
           ]
 
+        , testGroup "code blocks"
+          [ "identifier" =:
+            "\\begin{lstlisting}[label=test]\\end{lstlisting}" =?> codeBlockWith ("test", [], [("label","test")]) ""
+          , "no identifier" =:
+            "\\begin{lstlisting}\\end{lstlisting}" =?> codeBlock ""
+          ]
+
         , testGroup "citations"
           [ natbibCitations
           , biblatexCitations

tests/Tests/Writers/LaTeX.hs

@@ -10,6 +10,9 @@ import Tests.Arbitrary()
 latex :: (ToString a, ToPandoc a) => a -> String
 latex = writeLaTeX def . toPandoc
 
+latexListing :: (ToString a, ToPandoc a) => a -> String
+latexListing = writeLaTeX def{ writerListings = True } . toPandoc
+
 {-
   "my test" =: X =?> Y
 
@@ -31,6 +34,10 @@ tests :: [Test]
 tests = [ testGroup "code blocks"
           [ "in footnotes" =: note (para "hi" <> codeBlock "hi") =?>
             "\\footnote{hi\n\n\\begin{Verbatim}\nhi\n\\end{Verbatim}\n}"
+          , test latexListing "identifier" $ codeBlockWith ("id",[],[]) "hi" =?>
+            ("\\begin{lstlisting}[label=id]\nhi\n\\end{lstlisting}" :: String)
+          , test latexListing "no identifier" $ codeBlock "hi" =?>
+            ("\\begin{lstlisting}\nhi\n\\end{lstlisting}" :: String)
           ]
         , testGroup "math"
           [ "escape |" =: para (math "\\sigma|_{\\{x\\}}") =?>