Support DOIs natively and fetch citations

[dleehr]

Discussed having the DOI as a separate field for the references (and also getting the citation info from the DOI, if present).

[dleehr]

With the changes to how references are stored/parsed in #55, this is less of an issue.

If there is a DOI in the references field, it will be hyperlinked. However, the citation is not fetched.

[dleehr]

Marking as needs discussion, since this may not be necessary