improve arg pass through

[jmbarbone]

Should be able to let one arg be dependent on another

library(scribe)
ca <- command_args("-a")
arg <- new_arg("-a", action = "flag")
ca$add_argument(arg)
ca$add_argument("-b", default = arg)
args <- ca$parse()
print(args)
#> $a
#> [1] TRUE
#> 
#> $b
#> [1] TRUE