resolves #690 keep block image with caption; resolves #691 resize SVG to fit page (PR #692)
resolves #690 keep block image with caption; resolves #691 resize SVG to fit page (PR #692)

- keep block image on same page as caption
- calculate height of caption accurately using dry_run
- resize SVG to fit on page with caption if taller than content area of single page
- advance page manually instead of using keep_together to place SVG
- minor code cleanups
dsisnero/asciidoctor-pdf,dsisnero/asciidoctor-pdf,Hextremist/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,Hextremist/asciidoctor-pdf,asciidoctor/asciidoctor-pdf