Update Asciidoclet guide from latest asciidoctor/asciidoclet/README.adoc.
Add Asciidoclet 1.5.0 announcement.
Add an example of using asciidoctor-diagram with Asciidoclet.
Add note about --gem-path.
Changed --gem-path wording since it will soon be required when loading gems.

Also switched to callouts in the extension loading example arguments to
highlight --gem-path and data-uri options.
production edits for the Asciidoclet 1.5.0 release announcement
Merge PR #294 - Asciidoclet 1.5.0 release announcement