Support for Sidebars in Asciidoc Writer

[GHyman83]

First of all, thanks for creating and maintaining this amazing tool! I've been using the CLI for several years and just recently have gotten into working with the library and Lua filters, and it's been such a lifesaver for my work.

Last week, as I was writing a filter for generating sidebars, it occurred to me that the Asciidoc writer currently supports fenced admonitions (notes, tips, etc.) but, from what I can tell, not sidebars, which follow a very similar pattern.

Acknowledging that sidebars (as RawBlocks) can be generated from Divs by using filters, I was wondering if it wouldn't make sense to add support for sidebars to the Asciidoc writer, to keep them on similar footing as admonitions?

I took an initial stab at modifying the writer and adding a test (the latter not shown below). It seems to work well with the handful of test cases I've thrown at it. Would something like this be considered if I were to create a PR with the change to the writer and the test?

-- ...Writers/Asciidoc.hs
...
-- convert admonition and sidebar divs to asicidoc
blockToAsciiDoc opts (Div (ident,classes,_) bs) = do
  let identifier = if T.null ident then empty else "[[" <> literal ident <> "]]"
  let admonition_classes = ["attention","caution","danger","error","hint",
                     "important","note","tip","warning"]
  let sidebar_class = "sidebar"
  
  contents <-
       case classes of
         (l:_) | l `elem` admonition_classes || T.toLower l == sidebar_class -> do
             let (titleBs, bodyBs) =
                     case bs of
                       (Div (_,["title"],_) ts : rest) -> (ts, rest)
                       _ -> ([], bs)
             let fence = if l == "sidebar" then "****" else "===="
             elemTitle <- if null titleBs ||
                                   -- If title matches class, omit
                                   (T.toLower (T.strip (stringify titleBs))) == l
                                   then return mempty
                                   else ("." <>) <$>
                                         blockListToAsciiDoc opts titleBs
             elemBody <- blockListToAsciiDoc opts bodyBs
             return $ "[" <> literal (T.toUpper l) <> "]" $$
                      chomp elemTitle $$
                      fence $$
                      chomp elemBody $$
                      fence
         _ -> blockListToAsciiDoc opts bs
  return $ identifier $$ contents $$ blankline
...

[jgm]

Sure, go ahead and submit a PR!