Repeatable required option fails to parse with one value

[afranchuk]

Right from the README quick reference, trying to use repeatable(required("-l") & value("lines", ids)) fails to parse correctly, where the parsing_result considers the required value as a missing argument:

#include "clipp.h"
#include <iostream>

int main(int argc, char *argv[])
{
    using namespace clipp;

    std::vector<size_t> ids;

    auto cli = repeatable(required("-l") & value("lines", ids));
    auto result = parse(argc, argv, cli);
    if (!result) {
        debug::print(std::cerr, result);
        return 1;
    }
    return 0;
}

Result of running:

$ ./a.out -l 0
#1 -l -> -l 
#2 0 -> lines 
lines    [missing after 0]

Note that more than one argument (e.g., ./a.out -l 0 -l 1) parses correctly.