filters: date: add target option

Update the date filter to support a target option.  If defined,
instead of updating the @timestamp field, update the given target field.
This is useful for events that have multiple timestamps (e.g., start and
end times).
Merge pull request #625 from jdve/date-overwrite

filters: date: add target option
Merge git://github.com/logstash/logstash