Cross-compile asciidoctor-extensions-lab

Resolves #64 

To make it work:
 * Replace `require File.join File.dirname(__FILE__), File.basename(__FILE__, '.rb'), 'extension'` by `require File.join File.dirname(__FILE__), 'chrome-inline-macro/extension'` in `extensions-lab/lib/chrome-inline-macro.rb`
 * Remove `require 'asciidoctor'` and `require 'asciidoctor/extensions'` in `extensions-lab/lib/chrome-inline-macro/extension.rb`

TODO: iterate through a list of "endorsed" extensions