Teach logStash to optionally log to a local file

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