[gatsby-remark-copy-linked-files] support Asciidoc files

[gdams]

Hi there,

I've been using the gatsby-remark-copy-linked-files module very effectively with a markdown blog for some time now but have decided to migrate away from using Markdown and towards Asciidoc (using the gatsby-transformer-asciidoc module). I was wondering if there was a way that I could use the gatsby-remark-copy-linked-files module in the same way to copy images from their relative directories into a public folder?

[gdams]

CC @KyleAMathews as you appear to be the maintainer of the plugin