resolves #31 support attributes as String, List and Array

I implemented all the logic for processing the String-based and List-based attribute declarations. In the future, we could consider delegating this to Asciidoctor Java if it can be done safely. However, due to issues with GString, I'm guessing it's safest done in the plugin, or using a utility method provided by Asciidoctor Java (ideal).

The tests verify that the attributes are processed without exception, but not that they are processed correctly.