asciidoctorj cli should not hardcode the doctype

[mojavelinux]

The asciidoctorj cli should not hardcode the doctype to article by default. The default should be unset.

By setting the doctype attribute from the cli automatically, this prevents the document from being able to define the doctype. As a result, it's not possible to use asciidoctorj on a mixed set of documents (documents with different doctypes).

To demonstrate this problem, run the AsciidoctorInvoker with no flags defined on a document that defines its doctype as book:

= Title
:doctype: book

= Part 1

== Chapter 1

It was a dark and stormy night.

This will result in the following warning:

asciidoctor: ERROR: book.adoc: line 4: only book doctypes can contain level 0 sections