Support images with absolute paths

[someth2say]

AsciiDoctor documentation states that The value of imagesdir can be an absolute path, relative path or URL.

Nonetheles, using an absolute path causes the image to not be included in the ePub file.

Reproducer: If the following imagesdir.adoc file and the image.svg are in /tmp

= Book
:imagesdir: {docdir}

image::./image.svg[]

Causes raises the following error:

asciidoctor: ERROR: imagesdir.adoc: media file not found or not readable: /tmp/tmp/image.svg