v2.0: funtion to process results of downloads from MEDLINE

It is possible to download records from MEDLINE (e.g., with packages such as [`easyPubMed`](https://cran.r-project.org/web/packages/easyPubMed/easyPubMed.pdf)). A function that converts these to match the output of references_read() should be straightforward to implement. 