Teach logStash to accept static key/values from the config file

This PR gives the `logstash` writer the ability to write a static list of key-value pairs to each log entry.  This allows for custom configuration file to be written on different servers/environments that will contain static data to log, such as the server name, ip address, or other important server-specific information.

One way to leverage this change is to use the npm `config` module with a default bucker configuration section and override that using a separate config per environment and server.

* Example:
```
{
    "logstash": {
        "static_keys": {
            "hostname": "host.domain.com",
        }
    }
}
```