Allow multiple attributes to be specified

[mojavelinux]

It's possible to specify multiple attributes, but it's not consistent with how the asciidoctor commandline works. Right now, you have to separate the attributes by spaces:

asciidoctorjs -a 'icons=font linkcss' sample.adoc

Ideally, it should be possible to specify the -a flag multiple times:

asciidoctorjs -a icons=font -a linkcss sample.adoc

However, that depends on array support in the cli package (see node-js-libs/cli#62).

As an alternative, we should allow attributes to be separated by commas so that quotes are not needed:

asciidoctorjs -a icons=font,linkcss sample.adoc

(this syntax used to be possible in Asciidoctor core, but was dropped).

If no change is made to the code, we should at least document in the README how to specify multiple attributes since it is different from the asciidoctor command.

[ggrossetie]

As suggested by @mojavelinux we should switch to https://www.npmjs.com/package/yargs

[mojavelinux]

👍