Only use fixed image size when the backend is html5
Detect HTML backends using regexp rather than exact 'html5' string comparison
Add spec for omitting width/height in docbook output
Merge pull request #1 from pepijnve/fixed_image_size

Fixed image size
Merge branch 'master' of https://github.com/asciidoctor/asciidoctor-diagram into fixed_image_size

Conflicts:
	spec/plantuml_spec.rb
Merge pull request #32 from neher/fixed_image_size

Only generate width and height attributes when the backend is an html one