Add offline time update to NTPClient capability

[saifuhameed]

In my application NTPClient is used to get time for logging and displaying IOT events.
We expect device to display IOT data through local webserver even without internet.
But there is a problem, event time will be wrong due to there is no internet. So I suggest to add a function to enable offline updating of date and time, so that it will give at least actual date and time with reduced accuracy. Offline updating can be done from a settings page of IOT application, user can update date and time on device startup.
This feature can be easily accommodated by adding a function to update _lastUpdate and _currentEpoc manually.

[Walorikot]

You can update the current time manually through the offset:

  // Setup
  WiFiUDP ntpUDP;
  NTPClient timeClient(ntpUDP);
  // Update
  timeClient.setTimeOffset(currentEpoch - millis() / 1000);

The last part tries to compensate _lastUpdate. It is the best solution I've found so far and it is not accurate. There should be an easier way to do this. Hopefully this library will get this feature.