Add exception traceback handling to Formatter

[logston]

Hi. If I wrote a test for this patch, would you be interested in merging it in?

Thanks for your time.

[ngiallelis]

I think that formatted exception exists already in record.exc_text field. See logging.Formatter.format() function.
And it is called in line 47 of handler.py super(FluentRecordFormatter, self).format(record)

[logston]

Gotcha thank you.