Allow erb in config

Given the following i18n-js.yml:

```
translations:
<% Widgets.each do |widget| %>
- file: <%= "'app/assets/javascripts/i18n/#{widget.namespace}/%{locale}.js'" %>
  only: <%= "'#{widget.namespace}'" %>
<% end %>
```

I want "i18n-JS" to execute the included ERB to allow more magic.

This commit will implement this feature and change a minor 'stub' in
spec_helper.rb.
It should be `config/i18n-js.yml`.
Also should we mention it in README?