External services on chain providers & Perform no deep merging on providers.

### External services on chain providers

Before this PR, only providers with prefix “bazinga_geocoder.provider.“ are supported. Even if you prefix your service by it, you'll have to register your bundle before BazingaGeocoderBundle (see [here](https://github.com/geocoder-php/BazingaGeocoderBundle/blob/master/DependencyInjection/BazingaGeocoderExtension.php#L235).

```
bazinga_geocoder:
    providers:
        google_maps: ~
        chain:
            providers: [app.x.provider.foo, google_maps]
```

### Perform no deep merging on providers.

Providers are merged through configurations, it could be problematic, my use case:

```
# config.yml
bazinga_geocoder:
    providers:
        google_maps: ~
        chain:
            providers: [google_maps]
```

```
# config_test.yml
bazinga_geocoder:
    providers:
        chain:
            providers: [xxx.provider.stub]
```

Before this PR, `chain` providers will have  `[google_maps, xxx.provider.stub]` providers. 
After this PR, providers are not deep merged.