Function to_bibtex_with_fields 

pub fn to_bibtex_with_fields(
    citation_key: &str,
    m: &Metadata,
    extra: &[(&str, &str)],
) -> String

to_bibtex with extra fields appended after the ones the metadata supplies, in order – e.g. ("file", "refs/fock1930.pdf") (#610). Values get the same brace and markup scrubbing as every other field.