Function write_all_to_dir 

pub fn write_all_to_dir<D, P>(
    doc: &D,
    dir: P,
    stem: &str,
) -> Result<Vec<PathBuf>>
where
    D: ExportSource + ?Sized,
    P: AsRef<Path>,

Renders one document into every built-in format and writes the resulting files into dir using stem plus the format extension.