using cleanup, adding 'timestamp' field handling in the style of 'message'

[theor]

we use this appender to send exisiting log files to graylog, so we need to extract the timestamp from the log and use it instead of datetime.now.

[jjchiw]

Hi!.

I think this is kinda of specific, but anyway I'm going to merge it. Could you please write a Unit Test and update the readme.md to show the impact of this changes....

Thanks for the PR.....