Made --id-prefix work in DocBook as well as HTML.

Closes #607.

README

@@ -415,8 +415,8 @@ Options affecting specific writers
 
 `--id-prefix`=*STRING*
 :   Specify a prefix to be added to all automatically generated identifiers
-    in HTML output.  This is useful for preventing duplicate identifiers
-    when generating fragments to be included in other pages.
+    in HTML and DocBook output.  This is useful for preventing duplicate
+    identifiers when generating fragments to be included in other pages.
 
 `-T` *STRING*, `--title-prefix=`*STRING*
 :   Specify *STRING* as a prefix at the beginning of the title

src/Text/Pandoc/Writers/Docbook.hs

@@ -103,7 +103,7 @@ elementToDocbook opts lvl (Sec _ _num id' title elements) =
                  n | n == 0           -> "chapter"
                    | n >= 1 && n <= 5 -> "sect" ++ show n
                    | otherwise        -> "simplesect"
-  in  inTags True tag [("id",id')] $
+  in  inTags True tag [("id", writerIdentifierPrefix opts ++ id')] $
       inTagsSimple "title" (inlinesToDocbook opts title) $$
       vcat (map (elementToDocbook opts (lvl + 1)) elements')