Feature request: save images locally

[nathonius]

Currently the plugin links to the remote images from where they were slurped. It would be great to have an option to save the images alongside the markdown for eventual case where the url changes or the content is removed.

Since this is using almost the same stack as MarkdownDown and produces nearly identical output, I'm assuming you'll run into the same issue I created (from another GitHub account) for that project: https://github.com/asadm/markdowndown/issues/2

If this is out of scope for the plugin, I would look into using the hooks described in #37 to write a companion plugin myself. 😃