Changing default format from a more localized %d/%M/%Y to ISO 8601 standard

Default formatting of date in ISO 8601 Format
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This changes the default time/date from %d/%M/%Y to the internet standard for ISO8601 of %Y-%M-%d.  This is in response to [https://github.com/easylogging/easyloggingpp/issues/200](https://github.com/easylogging/easyloggingpp/issues/200).