fold wrapped list item text aligned with first line
fix missing nil check
add support for resolving reference and data image URIs

- add convenience methods for creating reference URIs to images
  relative to the asset dirs (imagesdir, iconsdir) (no disk access)
- add support for embedding images into the document when the data-uri
  attribute is set
- take extreme care to not allow access to ancestor directories when
  preparing to read an image from the filesystem
(re)implement callouts and callout lists

- also fix link inside angled brackets
use image_uri in docbook templates
Merge pull request #55 from mojavelinux/list-item-wrapped-text

Priority fix: fold wrapped list item text aligned with first line
Merge pull request #53 from mojavelinux/callouts

(Re)implement callouts and callout lists
Merge remote-tracking branch 'mojavelinux/embedded-images' into mojave

Conflicts:
	lib/asciidoctor/document.rb
Silly spaces