Infer the width and height from the generated image

[ggrossetie]

It would nice to infer the width and height from the generated image when width and height are not explicitly defined.
For reference, Asciidoctor Diagram does the following:

PNG
https://github.com/asciidoctor/asciidoctor-diagram/blob/66876e9c11c401eec943ff6968cd0c5f6c443906/lib/asciidoctor-diagram/util/png.rb#L9-L22

GIF
https://github.com/asciidoctor/asciidoctor-diagram/blob/66876e9c11c401eec943ff6968cd0c5f6c443906/lib/asciidoctor-diagram/util/gif.rb#L10-L18

SVG
https://github.com/asciidoctor/asciidoctor-diagram/blob/66876e9c11c401eec943ff6968cd0c5f6c443906/lib/asciidoctor-diagram/util/svg.rb#L7-L24

Originally posted by @Mogztter in #99 (comment)

[nekator]

Encountered this difference between asciidoctor-kroki and asciidoctor-diagram today as i replaced asciidoctor-diagram with asciidoctor-kroki. Missing width and height resulted in failing tests.
confluence-publisher/confluence-publisher@6500369#diff-46f69509d7d0c7f3b137dc0c21c7c8a787aba4a30ba7c527efa63173f1a9e8bcR1273