Treat div with class "notes" as speaker notes in slide formats.

Currently beamer goes to `\note{}`, revealjs to `<aside class="notes">`,
and the notes are simply suppressed in other formats.

Closes #925.

README

@@ -2731,6 +2731,24 @@ bibliographies:
 
     # References {.allowframebreaks}
 
+Speaker notes
+-------------
+
+reveal.js has good support for speaker notes.  You can add notes to your
+markdown document thus:
+
+    <div class="notes">
+    This is my note.
+
+    - It can contain markdown
+    - like this list
+
+    </div>
+
+To show the notes window, press `s` while viewing the presentation.
+Notes are not yet supported for other slide formats, but the notes
+will not appear on the slides themselves.
+
 Literate Haskell support
 ========================
 

src/Text/Pandoc/Writers/HTML.hs

@@ -421,9 +421,16 @@ blockToHtml opts (Para [Image txt (s,'f':'i':'g':':':tit)]) = do
 blockToHtml opts (Para lst) = do
   contents <- inlineListToHtml opts lst
   return $ H.p contents
-blockToHtml opts (Div attr bs) = do
+blockToHtml opts (Div attr@(_,classes,_) bs) = do
   contents <- blockListToHtml opts bs
-  return $ addAttrs opts attr $ H.div $ nl opts >> contents >> nl opts
+  let contents' = nl opts >> contents >> nl opts
+  return $
+     if "notes" `elem` classes
+        then case writerSlideVariant opts of
+                  RevealJsSlides -> addAttrs opts attr $ H5.aside $ contents'
+                  NoSlides       -> addAttrs opts attr $ H.div $ contents'
+                  _              -> mempty
+        else addAttrs opts attr $ H.div $ contents'
 blockToHtml _ (RawBlock f str)
   | f == Format "html" = return $ preEscapedString str
   | otherwise          = return mempty

src/Text/Pandoc/Writers/LaTeX.hs

@@ -285,7 +285,12 @@ isLineBreakOrSpace _ = False
 blockToLaTeX :: Block     -- ^ Block to convert
              -> State WriterState Doc
 blockToLaTeX Null = return empty
-blockToLaTeX (Div _ bs) = blockListToLaTeX bs
+blockToLaTeX (Div (_,classes,_) bs) = do
+  beamer <- writerBeamer `fmap` gets stOptions
+  contents <- blockListToLaTeX bs
+  if beamer && "notes" `elem` classes  -- speaker notes
+     then return $ "\\note" <> braces contents
+     else return contents
 blockToLaTeX (Plain lst) =
   inlineListToLaTeX $ dropWhile isLineBreakOrSpace lst
 -- title beginning with fig: indicates that the image is a figure