[HTML5 preview] Offline mode, use angular-directive for asciidoctor.js and opal.js

[mgreau]

@anthonny released an angularjs directive to play with Asciidoctor

https://github.com/anthonny/angular-asciidoc-directive
http://anthonny.github.io/ascii-demo/

It will be interesting to use this directive instead of the actual code.
This app uses an iframe in order to render the preview (HTML5...), the directive doesn't use an iframe for now.

[cmoulliard]

Should be good too to move to asciidoctorjs - 1.5.pre