Consider integration of std::optional (C++17)

Consider adding direct support for `std::optional` from C++17 (or `boost::optional`, etc).

Note that this is just for ease of use for people familiar with optionals, the functionality is already there:

```cpp
int option;
CLI::Option* option_ptr = app.add_option("-o", option);
// Parse

if(*option):
   // use option

// OR
if(option->count() > 0)
    // use option
```