take into account the outdir document attribute if specified
Merge pull request #31 from neher/issue30

Take into account the outdir document attribute if specified
Add test case for outdir attribute
Ensure image target directory exists before attempting to write file
Merge branch 'master' of https://github.com/asciidoctor/asciidoctor-diagram into fixed_image_size

Conflicts:
	spec/plantuml_spec.rb