Add options for rendering the DOI

[mjordan]

DOIs can be presented visually in various ways, e.g. screen and print, URI, URN, as documented at https://www.doi.org/doi_handbook/2_Numbering.html#2.6. Currently, the DOI is stored raw, like "10.1006/jmbi.1998.2354". We should provide at least the option to add 'doi' for display on the screen.

Note: this is a rendering issue. We should not prepend anything to the DOI value itself in storage.