feature request: merge annotations into pdf popup annotations

Loving this project!

It would be great if we could use pdf.js's annotation features to map hypothesis annotations to PDF annotations.

* This would be good (for example) just before a print, such that the PDF printer driver can make something nice out of the annotations.
* Another usecase is to share annotated PDFs via interfaces that expect annotated PDFs (journals and other types of editing environments).

Outline of the feature which would be a part of the "print" button, or an extra "export" button:
* maps normal annotations to pdf popup annotations
* optionally include group name and/or author name (good for anonymous reviews)
* requires a pop-up with options or pre-configured setting somewhere else
* maps page annotations to marging notes? free text. don't know what the best thing to do is
* same optional mentioning of group/author name
* maps highlights to highlights
* If author names are not to be included in the annotations, then this feature should guarantee the privacy
of the authors of the annotations. no meta-data should be included in the PDF that could reveal their identity.