Allow extensions to be enabled / disabled from an attribute

[mojavelinux]

Allow extensions (specifically extension groups) to be enabled and disabled using an attribute when running in safe mode < secure (or server). This feature is similar to how Markdown extension enablement works.

High-level vision:

$ asciidoctor -a extensions=ui,diagram,social

This would enable the extension sets "ui", "diagram" and "social" for the document. Of course, the extensions would still need to be registered. This feature is about activation. Extensions not listed and not registered as "auto-activate" would not be activated.

[mojavelinux]

Some additional discussion can be found in this thread on the discussion list: http://discuss.asciidoctor.org/Autoloading-extensions-tp1548p1562.html