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]' }
        ]
    }]
};
```
Sorry, of cause you're right … I tried to avoid the call to `Hoek. applyToDefaults` in each `timeString` call (got a bit distracted). This will be the best option, I think:

- Use the format parameter in `timeString`, apply to default value on each call
- Get custom format coming from `this._settings.format` in `_report`, set as second parameter for `printRequest` and `printEvent`.
- Change back the unit tests