asciidoc writer: translate numberLines attribute to linesnum switch

AsciiDoctor allows to request line numbering on code blocks by
using a switch on the `source` block, such as in:

```
[source%linesnum,haskell]
----
some Haskell code here
----
```

src/Text/Pandoc/Writers/AsciiDoc.hs

@@ -21,7 +21,7 @@ AsciiDoc:  <http://www.methods.co.nz/asciidoc/>
 module Text.Pandoc.Writers.AsciiDoc (writeAsciiDoc, writeAsciiDoctor) where
 import Control.Monad.State.Strict
 import Data.Char (isPunctuation, isSpace)
-import Data.List (intercalate, intersperse)
+import Data.List (delete, intercalate, intersperse)
 import Data.List.NonEmpty (NonEmpty(..))
 import Data.Maybe (fromMaybe, isJust)
 import qualified Data.Set as Set
@@ -193,7 +193,10 @@ blockToAsciiDoc _ (CodeBlock (_,classes,_) str) = return $ flush (
      then "...." $$ literal str $$ "...."
      else attrs $$ "----" $$ literal str $$ "----")
   <> blankline
-    where attrs = "[" <> literal (T.intercalate "," ("source" : classes)) <> "]"
+    where attrs = "[" <> literal (T.intercalate "," classes') <> "]"
+          classes' = if "numberLines" `elem` classes
+                        then "source%linesnum" : delete "numberLines" classes
+                        else "source" : classes
 blockToAsciiDoc opts (BlockQuote blocks) = do
   contents <- blockListToAsciiDoc opts blocks
   let isBlock (BlockQuote _) = True

test/Tests/Writers/AsciiDoc.hs

@@ -38,6 +38,22 @@ tests = [ testGroup "emphasis"
                para (singleQuoted (strong (text "foo"))) =?>
                  "`**foo**'"
           ]
+        , testGroup "blocks"
+          [ testAsciidoc "code block without line numbers" $
+               codeBlockWith ("", [ "haskell" ], []) "foo" =?> unlines
+                                           [ "[source,haskell]"
+                                           , "----"
+                                           , "foo"
+                                           , "----"
+                                           ]
+          , testAsciidoc "code block with line numbers" $
+               codeBlockWith ("", [ "haskell", "numberLines" ], []) "foo" =?> unlines
+                                           [ "[source%linesnum,haskell]"
+                                           , "----"
+                                           , "foo"
+                                           , "----"
+                                           ]
+          ]
         , testGroup "tables"
           [ testAsciidoc "empty cells" $
                simpleTable [] [[mempty],[mempty]] =?> unlines