Merge fallback locale

### Given the following "i18n-js.yml":

```
translations:
  - file: "public/javascripts/i18n/%{locale}.js"
    only: '*'
```

### Use-Case:

* Each generated JS-file should contain all localized strings. So we only need to include one js-file in frontent. This JS-File should containt all localized strings AND any fallback strings if localization is missing yet.

### Suggestion

```
translations:
  - file: "public/javascripts/i18n/%{locale}.js"
    only: '*'
    fallback: true
```

This will use `I18n.default_locale` as fallback.


OR

```
translations:
  - file: "public/javascripts/i18n/%{locale}.js"
    only: '*'
    fallback: "de"
```

This will use given locale as fallback.