Implement role attribute

[cunka]

Would like to see asciidoctor-tabs have a full role attribute implemented and working.
This would keep it additionally consistent with the way other blocks works.

• e.g. [tabs.chocolate]

https://docs.asciidoctor.org/asciidoc/latest/attributes/role/

Currently only the ID is working.

• e.g. [tabs#redWine]

Currently, If we lazily put a "." proceeding the name, (example: [tabs.]) then the terminal will report an empty role.

The idea behind using the role feature on a block is the ability to use a CSS style on 1 or more tabset(s).
Some branding styling guidelines mandate the use of a class ahead of an ID, simply for streamlining into an existing website. Having the use of a role makes the implementation easier.

[mojavelinux]

This seems like a reasonable request.

If we lazily put a "." proceeding the name, (example: [tabs.]) then the terminal will report an empty role.

That's normal behavior of Asciidoctor and is expected.