Add converter to std::chrono::duration

The converter allows you to work with config like this:
{
    "time1" : 2,
    "time2" : "3s"
}
in following way:
auto t1 = config.at<std::chrono::seconds>("time1"); // will be 2 seconds
auto t2 = config.at<std::chrono::milliseconds>("time2"); // will be 3000 milliseconds

So user can use convenient dimension of time, and developer can easily convert it to dimension he needs.