Add a default ctor to allow deferred parsing

[xparq]

Otherwise this couldn't work:

Args args (argc, argv); // or just Args args;

try {
    args.parse();  // nice and cosy place to catch errors!
    // or: args.parse(argc, argv);  // could also reset & reparse with other sets of args; could be handy!
} catch(...) {
    // boo!
}

if (args["flag"]) // use args...