Support custom timestamp format

Allow an optional argument to set a custom format for the log timestamp:

Example configuration for hapi:
```js
var options = {
    reporters: [{
        reporter: require('good-console'),
        args:[
            { log: '*', request: '*'},
            // ISO 8601 date format
            { timestampFormat: 'YYYY-MM-DDTHH:mm:ss.SSS[Z]' }
        ]
    }]
};
```
- Change signature of `printEvent` to take the event and the format
- Keep the `hoek.merge` logic so that each instance has a default format
- Pass in `event` and `this._settings.format`