Avoid to generate an empty block if the source is empty from the command line

[ggrossetie]

I'm using the following command to generate JavaScript code:

opal --compile -I lib -I asciidoctor -g asciidoctor -r asciidoctor/converter/docbook5 --dynamic-require ignore --no-opal --no-exit > build/asciidoctor-docbook5.js < /dev/null

It would be nice if I could tell Opal that there's no code to generate (apart from the docbook5 module).
A safe solution is to add an option --no-source to skip the generation.

[ggrossetie]

As suggested by @elia we could instead add a --module option to generate only the code of the module (inside an Opal module: Opal.modules["my_module"] = function(Opal) {)