Short captions for lists

Thanks for working on this extension! I would like to suggest adding a feature to use short captions in the generated list. This is useful if you have long explanations in your figure captions that you want to trim out in the list of figures. In LaTeX this is done [like so](https://tex.stackexchange.com/questions/11579/short-captions-for-figures-in-listoffigures). It seems like this would require some additional asciidoctor markup, so this might not be trivial to implement.