Loading external configs

Enable loading external configuration files through `a9n_require` config key.
Each loaded config would be available as a sub-config within main configuration, accesible via an accesor named after the loaded filename.

Examples:

```
# config/configuration.yml
defaults:
  a9n_require:
    - external_config
```

```
# config/external_config.yml
defaults: 
  external_key: 'external value'
```

Results:
```ruby

A9n.external_config.external_key
# => 'external value'

A9n.a9n_require
# => A9n::NoSuchConfigurationVariable
```