Feature 809: import Pubmed Central ID

Import "pmc" field into bibtex from pubmed/medline records.  This field is analogous to other pubmed identifiers (pubmed id, doi, pii) and is used in NIH grant applications when listing publications produced by NIH-funded work.

I was working on my grant progress report and realized this change was easy to make, so I went ahead and did it.  Since there was already a feature request, seems others are interested too.