Ability to Use sprintf Dynamic Config in Logstash-Output-S3 Plugin

[rcuza]

As a user I would like to be able to use sprintf dynamic configurations in the prefix configuration of the logstash-output-s3 plugin.

As stated by Magnus Bäck in a community discussion, this would be complicated to implement in a general way because of the complexities of storing the files before uploading them to S3. However a config like prefix => events/%{+yyyy.MM.dd} would only change once a day.

[ph]

I am closing this issue in favor of #4, so all the discussion can happen in the same place.
I have started on working on that feature.