Extend/improve mapping of BibTeX fields to CSL fields

[brechtm]

Resources:

• http://www.docear.org/2012/08/08/docear4word-mapping-bibtex-fields-and-types-with-the-citation-style-language/
• https://github.com/citation-style-language/schema/blob/master/csl-data.json
• https://github.com/citation-style-language/schema/blob/master/csl-citation.json

[jayvdb]

See #57 for one complexity of BibTex mapping to CSL.