resolves #536 implement svg inline

Most likely this might get merged after the upgrade to asciidoctor 2, at which point it will need some slight tweaks.

Implemented features: svg inline and interactive options, both for local and remote images. data-uri for svg, local and remote. Most likely, data-uri will work for png and other binary formats after the upgrade to asciidoctor 2.

I refactored the convert-document tests a bit to reduce duplication and clarify what is tested.

Both svg inline and data-uri need all antora references to be intercepted, even ones that are just the relative component of a ref spec. This involves changing html5.matchesResourceSpec. I think this is a good idea in any case as it now checks that the resource is present.