Add Asciidoctor attribute support

[mhogerheijde]

Fixes #156

Allows users to configure attributes for Asciidoctor. They are described here: https://asciidoctor.org/docs/user-manual/#attributes

[lightbend-cla-validator]

Hi @mhogerheijde,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[mhogerheijde]

I have since the initial check accepted the CLA. Is the check repeated automatically?

[mhogerheijde]

Can we re-check the CLA?

[ennru]

I checked the CLA manually and it's OK.

[lightbend-cla-validator]

At least one pull request committer is not linked to a user. See https://help.github.com/en/articles/why-are-my-commits-linked-to-the-wrong-user#commits-are-not-linked-to-any-user

[mhogerheijde]

I've included an en-passant update to latest version of AsciidoctorJ.

[ennru]

LGTM.

[ennru]

But now the latest commit has a different author, that's why CLA checking fails:

Author: Matthias Hogerheijde <matthias.hogerheijde@v**a.com>
Author: Matthias Hogerheijde <matthias.hogerheijde@e**i.nl>

Can you squash it and provide the correct author, please.

[mhogerheijde]

Ah, that's why.

I'll recommit using my personal git-id. Is there a standard way of doing work in company time and using the git-id from my company with the same Github user-account?

[mhogerheijde]

Oh, PEBCAK, just read the CLA stuff again for when it is corporate.

I'll contribute from my personal account 👍

[ennru]

Just came across this, sorry that it kept dangling for so long.