allow default values for positional arguments

[jmbarbone]

library(scribe)
ca <- command_args()
ca$add_argument("foo", default = "bar")
ca$parse()
#> $foo
#> [1] NA
ca$set_input("foo")
ca$parse()
#> $foo
#> [1] "foo"

^{Created on 2023-03-20 with reprex v2.0.2}