Allow to add extra fields

As suggested in https://github.com/Seldaek/monolog/pull/400#issuecomment-50885207, which would allow something like:

```
$webProcessor = new WebProcessor();
$webProcessor->addExtraField('xip', 'HTTP_X_FORWARDED_FOR')
    ->addExtraField('http_auth_user', 'PHP_AUTH_USER');
```