Add Transparent Comparator Support

It would be great of `boost::program_options::variables_map` could have [transparent comparator](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3465.pdf) support (c++14 feature), to potentially avoid unnecessary object creation when accessing values in the map. Theoretically only a `std::less<>` needs to be added as the third template parameter of the `std::map` `variables_map` inherits from. Currently:

````c++
// 1
boost::program_options::variable_map vm;
vm.find("test"); // implicit std::string object created and destoryed

// 2
std::string_view sv("test");
vm.find(sv); // does not compile

// 3
vm.find(std::string(sv)); // only way to inspect with a std::string_view, explicit std::string object created and destroyed
````
With a transparent comparator 1 will not create a `std::string` object and just compare directly against the c-string literal. 2 will work as shown without any extra objects being created. 3 will be unnecessary