Hello everyone.

I've decided to write a Lua filter for Pandoc on my own, something similar 
to scotthartley/pandoc-wrapfig 
<https://github.com/scotthartley/pandoc-wrapfig> with a slightly different 
syntax: it uses classes and attributes instead of modifying caption format.

![caption with **bold**](image.png "Image title"){#img .wrap width=3}

So far so good, but diving deeper in the details I've had some problems.

*Width attribute*

Quoting the manual (extension: link attributes 
<https://pandoc.org/MANUAL.html#extension-link_attributes>):

The width and height attributes on images are treated specially. When used 
> without a unit, the unit is assumed to be pixels.
>

and

Dimensions are converted to inches for output in page-based formats like 
> LaTeX. [...] Use the --dpi option to specify the number of pixels per 
> inch. The default is 96dpi.
>

However, PANDOC_READER_OPTIONS doesn't provide --dpi such option. I think 
it would be cool if Pandoc could handle it itself *before* calling the 
writers, or at least provide a conversion function within the API.

*Image with attributes and formatted caption*

Such syntax

![[caption]{.underline}](image.png)

produces the following output:

$ pandoc demo.md -t native
[Para [Image ("",[],[]) [Span ("",["underline"],[]) [Str "caption"]] 
("image.png","fig:")]]

$ pandoc demo.md -t latex
\begin{figure}
\centering
\includegraphics{image.png}
\caption{{caption}}
\end{figure}

$ pandoc demo.md -t html5
<figure>
<img src="image.png" alt="caption" /><figcaption><span class="underline">
caption</span></figcaption>
</figure>

I suppose it could be a bug, or maybe LaTeX doesn't support underline 
within the context of a caption. However I think some clarification is 
needed and the AST passed to Lua should be revised as well since it's 
pretty a mess:

![[caption]{.underline}](image.png)
1:
  c:
    1:
      1:
      2:
        1: underline
      3:
    2:
      1:
        c: caption

![caption](image.png)
1:
  c: caption

![*caption*](image.png)
1:
  c:
    1:
      c: caption

(*): these tables refer to el.c[2] only

Also, I couldn't find any good documentation on parameter's fields (at 
least for images) and it took me a couple of hours to understand it. I 
would add something like this to the docs:

-- Creates an image identical to the one given
function Image (el)
    local id, classes, attrs = unpack(el.c[1])
    local caption = el.c[2]
    local src, title = unpack(el.c[3])

    return pandoc.Image(caption, src, title, {id, classes, attrs})
end

Finally, this it the first filter I write (and my first experience with 
Lua) so I would really appreciate if the community could give me a feedback 
to check if I got everything right: current version is attached, thanks in 
advance!

I would really like to take this opportunity to thank John and all the devs 
for creating Pandoc: it's a unique piece of software, couldn't wish any 
better!

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