Feature: Interpret absolute image links relative to `g:mkdp_images_path`

Some common workflows interpret absolute links relative to a well-known directory rather than the actual root of the filesystem, akin to how a web server interpret absolute links relative to the domain. I don't think there is currently a way for this plugin to behave like that regarding images.

I would suggest an additional boolean option for interpreting absolute image links relative to the existing `g:mkdp_images_path` (if provided). For instance, if `g:mkdp_images_path` is set to `/foo/bar`, then an image like `` would resolve to `/foo/bar/baz.png`.

This should be an easy add-on and would turn this plugin from "great" to "excellent" :)