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

I realize this isn't documented anywhere, but...

We're trying to avoid adding new APIs where the document ID is optional. Instead, it should be a required parameter, and an exception should be raised (or a promise rejected) if the document ID isn't a number.

Also, `integer` isn't a JSDoc type. Please use `number`.

Also, please add an `@return` to the JSDoc.

So, at the beginning of the function you could do something like:

```javascript
documentId = parseInt(documentId, 10)
if (!isFinite(documentId)) {
  return Q.reject("documentId must be a number in call to _getSVGOMG");
}
```