HTML writer: support speaker notes in dzslides.

With this change `<div class="notes">` and also `<div class="notes"
role="note">` will be output if `-t dzslides` is used. So we can
have speaker notes in dzslides too.

Thanks to maybegeek.

src/Text/Pandoc/Writers/HTML.hs

@@ -458,6 +458,7 @@ blockToHtml opts (Div attr@(_,classes,_) bs) = do
      if speakerNotes
         then case writerSlideVariant opts of
                   RevealJsSlides -> addAttrs opts' attr $ H5.aside $ contents'
+                  DZSlides       -> addAttrs opts' attr $ H5.div $ contents'
                   NoSlides       -> addAttrs opts' attr $ H.div $ contents'
                   _              -> mempty
         else addAttrs opts attr $ H.div $ contents'