Positional args can again intermix with options

Merged Alan De Smet requested to merge argparse-2 into master

The move from optparse to argparse caused a regression: positional arguments could no longer intermix with optional arguments. This fixes that. (As of Python 3.7, parse_intermixed_args solves the problem, but we want to support 2.6, maybe 2.5.)

Should deal with issue raised in #15

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