support of `asctime`

[jbernardes]

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