In the file lib/request.js, currently there are only methods to get metadata from a journal paper from its URL or DOI. However, you can also find information using another identifier, PMID. You can use the PubMed API to find a paper's DOI given its PMID and therefore its url: http://www.ncbi.nlm.nih.gov/pmc/tools/id-converter-api/
Hint: use the "request" library: https://www.npmjs.org/package/request
Bonus points: Method that makes request from the other IDs available from the PubMed API.