Include citation_pdf_url as an alternate related target to associate for annotations

## Feature Request Form
### Problem you are trying to address with this feature

HTML and PDFs can be related together if we know the PDF URL. Often publishers stipulate these. meta name="citation_pdf_url" is one extremely commonplace approach.

### Your solution
Include citation_pdf_url as one of the tags we automatically associate with the current page URL.
For instance, here view-source:http://link.springer.com/article/10.1007/s13317-016-0090-1

Start by inspecting how the product currently discovers metadata tags and associates related URLs, then use the Springer example in the issue to verify the HTML metadata. Done means a page's citation_pdf_url is recognized as an alternate related target for annotations.

Written by the indexing model from the issue text.