Add a --watch option

[ggrossetie]

$ asciidoctorjs source.adoc -o dest.pdf --watch

The above command will watch "source.adoc" for changes.
It may be a bad idea because users might expect the --watch to be smart and also watch for included files or resources (images...). So in the end it might be hard to provide something useful without knowing the workspace.

Another idea is to configure --watch:

$ asciidoctorjs source.adoc -o dest.pdf --watch 'includes/**.adoc' --watch 'images/*'

We could also decide to go one step further and add browser sync to automatically reload the browser.

$ asciidoctorjs source.adoc -o dest.html 

In this case, if I edit source.adoc, my browser will automatically refresh the content when I save my changes.

If we decide that it's not the job of the Asciidoctor CLI to provide such feature then we should document/explain how to setup live reload (or even provide a Node.js package that integrate with the CLI)