Add converter to std::chrono::duration

The converter allows you to work with config like this:
```
{
    "time1" : 2,
    "time2" : "3s"
}
```
in following way:
```c++
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.

It is good point. I am going to remake config_time_parse_error_t to present error message as follows: `[exception-type: ]what-message; help-message`

Samples:
- in case of stoll's exception
`invalid_argument: ...(the string from invalid_argument::what() here); the time is expected to be represented as ... and so on`

- in case of simple fail
`invalid suffix;  the time is expected to ...`