Fix warning on clang

[pavelosipov]

Fix the following warning:

args.hpp:337:55: warning: implicit conversion loses integer precision:
      'std::__1::vector<args::argument, std::__1::allocator<args::argument> >::size_type' (aka 'unsigned long') to
      'std::__1::unordered_map<std::__1::basic_string<char>, int, std::__1::hash<std::__1::basic_string<char> >,
      std::__1::equal_to<std::__1::basic_string<char> >, std::__1::allocator<std::__1::pair<const std::__1::basic_string<char>,
      int> > >::mapped_type' (aka 'int') [-Wshorten-64-to-32]
        if (arg.flags.empty()) lookup[""] = arguments.size();

[pfultz2]

Thanks!