add optional parameter to getDocumentInfo to override flags

Allows you to override the default flags for getDocumentInfo. So, you can request a minimal set of document info with something like:

```javascript
g.getDocumentInfo(null, {
    compInfo:           false,
    imageInfo:          false,
    layerInfo:          false,
    expandSmartObjects: false,
    getTextStyles:      false,
    selectedLayers:     false,
    getCompSettings:    false
});
```

cc @anirudhsasikumar