Sample lua custom writer fixes (mostly editorial)

Here are some comments on the v2.14.1 sample lua custom writer. Please let me know if I should create separate issues for any of them.

Attribute sorting: attributes(attr) uses pairs(attr), which I believe means that the attribute order is unpredictable (not a problem per se, but annoying if ever comparing the output from multiple runs). Probably it should do whatever the Haskell HTML writer does?
Link attributes: Link(s, tgt, tit, attr) ignores the attr argument.
Image versus figure: CaptionedImage(src, tit, caption, attr) doesn't check for empty captions. I'm not sure about this one: I did hit it in real life, but it's possible that it was caused by a lua filter fiddling with the caption.
Unintentional head global? Table(caption, aligns, widths, headers, rows) can update a global head variable that doesn't appear to be declared or used anywhere. Probably it can just be removed?

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