Enabling more features of GraphViz through the GraphViz Macro

I'd like to be able to use more features from GraphViz via the GraphViz Macro.
The most important would be to be able of using SVG images.
So I've altered it and I'm submitting the patch with this issue.

The changes were mainly to the GraphViz macro, but the plugin also received an extra method.

By the way, why isn't this Macro documented at the Macro pages? It's an interesting one. I hope the Javadoc I've added to the class can help with that.