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.

2 files changed, 8 insertions, 3 deletions

diff --git a/src/Text/Pandoc/Readers/LaTeX.hs b/src/Text/Pandoc/Readers/LaTeX.hs
index ded57df5a..b785a9852 100644
--- a/src/Text/Pandoc/Readers/LaTeX.hs
+++ b/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 (/='}'))
diff --git a/src/Text/Pandoc/Writers/LaTeX.hs b/src/Text/Pandoc/Writers/LaTeX.hs
index ab579a326..bf056001f 100644
--- a/src/Text/Pandoc/Writers/LaTeX.hs
+++ b/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