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

include/boost/program_options/detail/parsers.hpp の引用された行から始め、create() から options() に一時オブジェクトを渡す、提供された例を実行します。options() と positional() について提案されているコピー方式と非 const 参照方式を比較し、選択した動作によってダングリング参照が発生しなくなっているか、または必要なライフタイムが明確に文書化されていることを確認します。

索引モデルが issue の本文から書いたものです。