Allow custom alice providers

Hello guys.

Just a little idea to allow custom alice providers.

```yml
#src/App/Resources/config/services.yml
services:
    # ...
    alice.provider.youtube:       # Provider is automatically added to Alice
        class: App\Alice\Provider\YoutubeProvider
    alice.provider.custom:        # Provider is automatically added to Alice
        class: App\Alice\Provider\Custom
```