Add Rails::Application#config_for

This is a convenience for loading configuration for the current Rails
environment.

```yaml
production:
  url: http://127.0.0.1:8080
  namespace: bc3_production
development:
  url: http://bc3.dev/cleversafe
  namespace: bc3_development
```

```ruby
Cleversafe.config = Application.config_for :cleversafe
```

cc @dhh
Might be clearer to read if this was called something else? As it is, the test later uses 'custom' in two places to refer to different things...