Batch Parameter Pre-registration

The main changes:

• Adds add_params({...}) method for batch pre-registration of options as parameters.
• Since pre-registration has to be done before parsing, we might as well just use the ctor, so adds new ctor for batch pre-registration.
• Adds begin() and end() for directly using range-for over positional args:

   for (auto& pos_arg : cmdl)
     cout << '\t' << pos_arg << '\n';