Analyse other bibtex parsers

[jayvdb]

In #18 , bibtexparser (https://github.com/sciunto-org/python-bibtexparser) was suggested as an better parser, and several problems with it were highlighted.

As that package is actively maintained, it would be good to raise those problems with python-bibtexparser, as they may be interested in solving them, and solutions from citeproc-py might be able to be integrated into their package.

And maybe there are other bibtex parsers which could be used.