Bibtex: support url and doi fields

[jayvdb]

TODOs:

• [ ] add testing to ensure correct support

[brechtm]

The problem is that the BibTeX spec (documentation) lists only a small number of fields. Most seem to have been added afterwards in and ad-hoc manner. So, before, I was reluctant to mappings for BibTeX fields to CSL fields (in the BibTeX class) because there is no official list of BibTeX fields.

Since there is never likely to be an official, complete BibTeX reference, we might as well add mappings as we encounter new BibTeX fields...