allow NA for action = "flag"

[jmbarbone]

NA should be accepted as a flag value -- which should represent a flag that is not explicitly stated

ca <- command_args()
ca$add_argument("--foo", action = "flag")
ca$parse()
#> $foo
#> [1] FALSE

ca <- command_args()
ca$add_argument("--foo", action = "flag", default = NA)
ca$parse()
#> $foo
#> [1] NA