experimental SVG support

Adds a feature flag "svgomg-enabled" that directs SVG exports at an experimental SVG export engine: https://github.com/adobe-research/svgObjectModelGenerator

We are making a lot of changes to svgOMG right now, so it is convenient to be able to update it without making more PRs here.  I created a branch in svgObjectModelGenerator called "generator-assets" and pointed the package.json there.  This gives us a bit more control.  I think if/when this moves out of experimental mode we should move to explicit labels or something.