Allow embedding `svg` directly into the HTML document

[multimeric]

At the moment, pandoc -t html -F mermaid-filter -o output.html input.md always outputs the diagram inside an <img> element. It might be nice to add a setting that puts it into an <svg> element so it can be affected by CSS and JS on the page.

[raghur]

Can you try out master branch? One issue I see is that the way mermaid generates ids breaks theming (last one wins)