Add support for table striping in docbook converter

[snaggen]

This patch adds support to have tables striped in the docbook converter, on par with the html converter.

[mojavelinux]

I do not want to hard code colors into the DocBook output. If DocBook provides an attribute that instructs the backend to add striping, then I would consider it. Otherwise, this is not an feature I am willing to consider.

If it is something you need, you can extend the converter to add it (see https://docs.asciidoctor.org/asciidoctor/latest/convert/custom/#extend-and-replace-a-registered-converter).

[snaggen]

Closing this. Will submit a combined patch for both header style and table