Handle argparse `%(default)s` placeholder (and others?)

[jimporter]

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argparse has a nice feature where you can specify %(default)s in the help string, and it will fill in the default value as specified by the default keyword arg. However, shtab doesn't seem to be aware of this, so it appears literally when shell-completing. It would be nice if shtab could interpolate this value when generating the shell-completion functions.