Add Transparent Comparator Support

[joesdiner]

It would be great of boost::program_options::variables_map could have transparent comparator 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:

// 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