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

Should probably do `documentId = parseInt(documentId, 10)` before calling `isFinite`, since `isFinite("1") === true`. IDs often get added as keys in objects (for maps of IDs o Documents, for example), which stringifys them. 

I'll change this.