Implement export of bibdata with bibtex strings

[omangin]

#165 implements the representation of bibdata where values use bibtex string combinations. The writer however does not implement how to export such data.

Converting the BibDataStringExpression to bibtex should be straightforward but some logic should be implemented to decide which string definitions to include in the output bibtex file. Choices could be:
– all defined strings,
– only the strings used in the bibdata attributes,
– all strings except the common ones,
– all but an excluded list passed as parameters.

[MiWeiss]

A corresponding middleware (which replaces values with string references, according to a selectable strategy as described above) is still missing in v2.

It's currently not on my personal todo list, but I am happy to review and merge a corresponding pull request, or to list an "external middleware" in the docs.