--help command line argument

[ghost]

Although asciidoctor-pdf it's fairly simple to use, I don't like the error I get when I run asciidoctor-pdf --help xD.

[mojavelinux]

The plan is to integrate this converter as a proper Asciidoctor backend, in which case you'd use the asciidoctor command to invoke it. If we do decide to keep the asciidoctor-pdf command, we need to switch to using a proper CLI framework, at which point we'd surely add a --help flag.

[mojavelinux]

This will be resolved by #32.