Adding a preProcess config option

This is a small feature which lets you manipulate the log with a custom function before its pushed out.

We've used it like this : 

<code>
manifest.plugins['hapi-bunyan-lite'] = {
  logger: myRootLogger,
  defaultLogLevel: 'trace',
  preProcessData: function preProcessData(objLog, event, tags, request) {

    if (typeof event.data === 'string') {
      event.data = {
        msg: event.data
      };
    };

    if (request) {
      objLog.remoteAddress = request.remoteAddress;
      objLog.host = request.host;
      objLog.path = request.path;
      objLog.method = request.method;
      objLog.received = request.info.received;
      objLog.msec = new Date().getTime() - request.info.received;
      objLog.isAuthenticated = request.auth.isAuthenticated
    }
    
  }
};
</code>

Which makes the logs a joy to work with in http://www.elasticsearch.org/overview/kibana/ and lets us trace down bottle necks.