support of `asctime`

It would be great to support both milliseconds and timezone in the datefmt, for example: `%Y-%m-%dT%H:%M:%S,%f%z`. (I used `%f `for milliseconds, even though it's not supported by` datetime.strftime`.)

I understand this limitation comes from the standard library formatter `logging.Formatter`, but if `fluent-logger-python` could work around or resolve it, that would be very helpful.

As a work around, user may provide a `format_time(record, datefmt)` function to the constructor instead of relying on the `logging.Formatter.formatTime`