Zeek log files that have been renamed and are in the process of moving not caught correctly by Logstash

[mmguero]

Thanks to @rsatrio, see cisagov#221.

Sometimes Zeek renames a file before moving it, and Filebeat/Logstash can catch it before it's moved but after it's renamed. These files have a datestamp in them, which used to be like conn.2020-01-16-14-00-00.log but has at some point more recently been changed to conn.2020_01_16_14_00_00.log. I've updated the parsing to handle both (see mmguero-dev@c8f7303). It should now handle _ (underscore), - (dash), : (colon) or `` (no character) as separators for the date.