Specify creator sub-role in names data

[bwiernik]

See #293

Is your feature request related to a problem? Please describe.
Sometimes, one might want to indicate a more specific or custom role than the general label for a creator (e.g., to indicate "cartographer" or "speaker" instead of no label for an author).

Related to PR #337 and (see "localized terms in input data" #240)

A specific use case in a style guide in linguistics:
https://forums.zotero.org/discussion/84042/tromso-recommendations-for-citation-of-research-data-in-linguistics#latest