Option with empty argument causing TypeError

[jblakeman]

Arguments -t/--top-number and -m/--min-status fall back to default values when the options are not present on the command line. However, when they are passed without a following argument, None is stored and causes the following error:

TypeError: unsupported operand type for long(): 'NoneType'

Use the const keyword argument to argparse.add_argument, storing the option's default value to handle those cases.