Add support for repeating positional arguments.

[fabjan]

Usage: hello <foo> <bar> <baz>... should be allowed.

When configured and used like so:

...
    use bazz, cmd <- arg.strings("baz")

    try bazz = bazz(cmd)
...

bazz should be a Result with a list of strings.

arg.bools, arg.floats, arg.ints, arg.strings should be added, to parse lists of booleans, floats, integers and strings. The list consists of all the following non-option arguments in ARGV.