pre-define some common abbreviations

[eigenform]

Zola uses this crate. I noticed that some citations in Zotero are exported with a month tag, expecting nom-bibtex to recognize abbreviations for the names of different months.

This creates a set of default string expansions (variables defined in the parsed input take precedence).

[charlesvdv]

Those values are not defined in bibtext itself but in the style (.bst file) that are inputted in bibtext at parsing time. Since this parser does not support .bst format and I doubt it will be in the near future... I guess having some sane default should be fine. If ever someone is not fine with those defaults, we can always expose another method without any variable replacement.

Thanks for your contribution!