DOI content negotiation instead of calling CrossRef directly

[lawlesst]

Related to #4. This twitter thread indicates to that by using content negotiation against DOIs you can get back metadata regardless of the registration agency.

I tried 10.6083/M4MW2FFB from #4 and was able to get back metadata in JSON. If orcid2vivo implements or uses this function for fetching DOI metadata, then the registration agency doesn't matter.

This doesn't resolve the issue of sparse metadata but does allow for fetching metadata for DOIs from datacite without having to write additional code.