Timestamp override

[purbon]

From #7 that has been closed due to a long stalled CLA request not fulfilled.

Added option to specify timestamp from field in event.
use the timestamp_override field to define the field to use when specifying a timestamp instead of using the timestamp of the event. Useful if parsing historical log files.

[pandaadb]

This would be really cool! I looked at solving that issue yesterday since I backparsed a few weeks.

For what it's worth, there is a workaround:

Logstash seems to be executing filter in sequence of appearance. I managed to mutate the event timestamp to contain the log timestamp. That way all back-parsed logs had no issues.
What I really want to do though, is to up-vote and say how cool this feature would be :)

[codiply]

I opened PR 28 for this enhancement. What is the process for getting the PR reviewed?