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

[slonopotamus]

There are multiple issues here.

First, our logic in register_media_file is broken:

        if Asciidoctor::Helpers.uriish? target
          # We need to add both local and remote media files to manifest
          fs_path = nil
        else
          out_dir = node.attr('outdir', nil, true) || doc_option(node.document, :to_dir)
          fs_path = (::File.join out_dir, target) <-------------------------
          unless ::File.exist? fs_path
            base_dir = root_document(node.document).base_dir
            fs_path = ::File.join base_dir, target
          end
        end

Second, asciidoctor-epub3 assumes that "virtual" path (that is specified in AsciiDoc source) can be used as-is inside epub file. But this is not true in your case. Looks like we need to generate some path that would be used inside epub file.