Teach logStash to optionally log to a local file

[cjthompson]

This change leverages the logstash writer to write to a log file using the logstash format.

This change allows for a file key to define an output log file that will be written to independently of the redis or udp settings.

• Example usage:

{
    "logstash": {
         "file": "/var/log/app/logstash.log"
    }
}