Add setting to allow automatic figure number to be based on chapter number

[rlopez133]

Currently, when adding an image and providing a title it numbers a Figure as Figure 1. <Title>, Figure 2. <Title> etc, it be great if it would label the Figure based upon its chapter. Assuming you have two images in Chapter 2 and one image in Chapter 3, it would be Figure 2.1 <Title>, Figure 2.2 <Title>, Figure 3.1 <Title>, etc.

[mojavelinux]

Technically, this issue needs to be resolved in core. However, we can experiment with a solution here and then migrate it upstream once we're happy with the approach.

[mojavelinux]

See asciidoctor/asciidoctor#2291

[mojavelinux]

Here's a super quick extension to insert the section number after the figure caption and before the figure number.

Asciidoctor::Extensions.register do
  tree_processor do
    process do |doc|
      figcaption = doc.attr 'figure-caption'
      doc.find_by context: :image do |image|
        next unless image.title?
        section = parent_section image
        fignum = image.attributes[:attribute_entries].find {|it| it.name === 'figure-number' }.value
        image.caption = %(#{figcaption} #{section.sectnum} #{fignum}. )
      end
    end
    nil

    def parent_section block
      ancestor = block
      while (ancestor = ancestor.parent).context != :section
      end
      ancestor
    end
  end
end