removes sourceHighlighter mojo configuration in favour of attributes

[abelsromero]

This PR:

• Removes the sourceHighlighter configuration property
• Updates tests accordingly
• Updates README accordingly

[coveralls]

Coverage decreased (-0.06%) to 69.659% when pulling 365a067 on abelsromero:refactor/remove-sourceHighlighter-configuration into f55a6ab on asciidoctor:master.

[nagromc]

To those who may be wondering how to migrate to the new way of highlight their snippets, check out this page.

The README.adoc has been updated, but not the official webpage. Caused me head scratching for an hour 😅

[abelsromero]

Thanks for the heads up! Since you found it, you can create a PR to fix the url in https://github.com/asciidoctor/asciidoctor.org/blob/576efc0243b824c5f53f25ff39ac66c3f7ea732b/docs/asciidoctor-maven-plugin.adoc. I think we only need to fix the version 1.5.7.1 by 2.0.0.