pdf2csv: errors handling + formatting args on the CLI

feature: formatting args to the CSV writer from the CLI
    
    This permits to call the script with formatting options to be passed to the CSV writer.
    Use the form "name=value" after the pdf file name.
    For example, to change the delimiter:
        ./pdf2csv.py my_file.pdf delimiter=@
    May not work with arguments expecting something different than a string.

bugfix: do not crash on empty lists
bugfix: catch decode error and write "?"s instead