DocBook reader: honor linenumbering attribute

The attribute DocBook linenumbering="numbered" attribute on code blocks
maps to "numberLines" internally.

src/Text/Pandoc/Readers/DocBook.hs

@@ -907,6 +907,7 @@ parseBlock (Elem e) =
            let classes' = case attrValue "language" e of
                                 "" -> []
                                 x  -> [x]
+                ++ ["numberLines" | attrValue "linenumbering" e == "numbered"]
            return $ codeBlockWith (attrValue "id" e, classes', [])
                   $ trimNl $ strContentRecursive e
          parseBlockquote = do

test/docbook-reader.docbook

@@ -101,6 +101,11 @@
 sub status {
     print "working";
 }
+</programlisting>
+    <programlisting linenumbering="numbered">
+sub status {
+    print &quot;working with line numbers&quot;;
+}
 </programlisting>
     <screen>
 % <command>ls</command>

test/docbook-reader.native

@@ -266,6 +266,9 @@ Pandoc
           ]
       , CodeBlock
           ( "" , [] , [] ) "sub status {\n    print \"working\";\n}"
+      , CodeBlock
+          ( "" , [ "numberLines" ] , [] )
+          "sub status {\n    print \"working with line numbers\";\n}"
       , CodeBlock ( "" , [] , [] ) "% ls"
       , Para [ Str "A" , Space , Str "list:" ]
       , OrderedList