Provider-aware install generator

Given a command like: `bin/rails courrier:install --provider=loops` would create:

```
Courrier.configure do |config|
  config.default_email_provider = "loops"
  config.default_email_api_key = ""

  config.providers.loops.transactional_id = ""
end
```

Similarly for other providers, their specific values are added (if any).