Add PubMed as a data source

[Samwalton9]

Right now we're only looking at retracted papers as reported via CrossRef. We should also use PubMed.

As far as I can tell, however, it's much less straightforward to query for retractions. We can search for terms like "Retracted Publication", but that also finds papers about retracted publications. Not sure if there's a way that's as simple as CrossRef's data type = 'retraction' search.

[Samwalton9]

https://github.com/neilfws/PubMed/tree/master/retractions might be of use.