Stub config_file in spec_helper instead of config

In spec_helper.rb#set_config(path) the config is stubbed.
We should only stub the config_file (path) instead, so we don't have to
duplicate the code for YAML-parsing.
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.
Add erb-hint in CHANGELOG.md
Add erb-hint in README.md
Merge pull request #224 from Deradon/allow-erb-in-config

Allow erb in config