Get non-const reference for option value if needed

Now it is possible to get non-const (read-write) reference to underlying
options value storage variable. This is useful if you want to modify some option
in place and store it back to cmdline::parser.

### Reasoning

If I want map some variables, so the modifications are reflected and stored back into `cmdline::parser`, I am using references in my code, for example:

```c++
  auto &n_pixels    = cl.get<size_t>("n-pixels");
  auto &n_detectors = cl.get<size_t>("n-detectors");
  auto &n_emissions = cl.get<size_t>("n-emissions");
  auto &radious     = cl.get<double>("radious");
  auto &s_pixel     = cl.get<double>("s-pixel");
  auto &w_detector  = cl.get<double>("w-detector");
  auto &h_detector  = cl.get<double>("h-detector");
```

Now any changes to these variables will be takes when doing serialization, see my **Serialization** pull request.