basic_command_line_parser::options() may store address of temporary

Hi,

I've stambled accross a bug in my application about unrecognized options. After several minutes and checks I've realized it comes from Boost Program_Options because the `basic_command_line_parser::options` function store the address of a const-reference passed as argument. And since temporaries can bind to const-reference the library did use a dangling pointer.

It is not mentioned in the [documentation](https://www.boost.org/doc/libs/1_69_0/doc/html/boost/program_options/basic_command_line_parser.html) that user must specify a valid reference.

I propose that options() and positional() make a copy (as it's backward compatible) or to take a non-const reference as it explains correctly the ownership to the caller (but not backward compatible)

Example of code that may reproduce the bug:

```cpp
#include

namespace po = boost::program_options;

po::options_description create()
{
po::options_description desc;

desc.add_options()
("verbose", po::bool_switch());

return desc;
}

int main(int argc, char** argv)
{
po::variables_map vm;
po::store(po::command_line_parser(argc, argv).options(create()).run(), vm);
po::notify(vm);
}
```

Offending code https://github.com/boostorg/program_options/blob/develop/include/boost/program_options/detail/parsers.hpp#L37

Commencez par include/boost/program_options/detail/parsers.hpp à la ligne citée et exécutez l’exemple fourni, qui transmet un objet temporaire de create() à options(). Comparez les approches proposées par copie et par référence non const pour options() et positional(), puis vérifiez que le comportement choisi n’autorise plus une référence pendante ou documente clairement la durée de vie requise.

Rédigé par le modèle d'indexation à partir du texte de l'issue.