WIP: Add support for PDF url and fingerprint detection

1.

Now the PDF plugin is able to properly detect the
URL and fingerprint of the PDF document being viewed
by PDF.js.

However, this support is not leveraged by Annotator core,
for now. (It's up the the application consuming Annotator
to activate it.)

Also, the current implementation should be amended
as https://github.com/hypothesis/h/issues/1407 is fixed.

(The pending change will break compatibility of the URL scheme,
so annotataions linked to PDF fingerprints will be LOST.)

2.

Add minimal metadata extraction ability to the PDF plugin

From now on, the Document plugin is not supposed to be loaded,
when using the PDF plugin.

The PDF plugin is now replacing this functionality for PDF.
(Currently, it can only extract URLs and title, but that's all
we have or need for now.)

3.

Migrate the PDF plugin to use native Promise

With this, we stop using the promise implementation
bundled with PDF.js.

Solves #69.

  * * *

Don't deploy this yet.