Add back PDF.js to Chrome extension

This commit contains all the actual changes to H,
which are required for the new PDF support.

1. Use different insertion method.

Old method:
 - First PDF.js does it's magic, and redirects the page from the
   original document to it's own HTML content (the viewer)
 - Then embed.js injects the Hypothes.is code to the HTML page.

New method:
 - We embed our code directly in the viewer

Details:
 - I created a fork of viewer.html, the only change being
   that it also includes a reference to our embed script.
   PDF documents are rendered using this viewer
 - I have created a different embed.js version, called embed-pdf.js,
   tailored for this specific use-case. The main difference is that
   it actually uses eval() to run the code, instead of inserting a script
   tag. This was necessary because Chrome really, really hates inline
   JS code in extensions, and it's probably impossible to configure
   a CSP which would allow this.

2. Don't load the Document plugin for PDF files

3. Add support for proper URL and metadata extraction for PDF