PPTX: Expose comments

Created by: Anonymous

Original issue 448 created by @ysavourel on 2015-03-02T22:28:31.000Z:

It's possible that this enhancement might cover all OOXML file types if it's implemented generically; it was specifically requested for PPTX, so that's how I filed it.

The attached PPTX file has two comment annotations attached, one on the title slide and one on the body slide. These are not produced for translation. I've had a user request for exposing these, although I'm not entirely sure about the use case. This would need to be optional behavior (and I think disabled by default), as not all users would want to take advantage of this.