Preserve timezone info

[pznamensky]

Currently the plugin sends messages with date in UTC both for RFC3164 and RFC5424 formats.
But in syslog world messages usually sending after applying a timezone.
In fact there is a little bit messy when whole syslog server uses local time but messages received from logstash are in UTC and with missed TZ info.
Is it possible to emit messages after applying TZ and preserve TZ info?
Before:
2018-02-28T14:55:34.706+00:00
After:
2018-02-28T10:55:34.706-04:00
I guess it would break current setups with RFC3164 format because there is no TZ info in this format.
So maybe there is no necessity to change behavior for this format.
Nevertheless messages in RFC5424 could use timezone seamlessly. Maybe it would be reasonable to add flag like apply_timezone to clarify this behavior.