supporting whitelist of langs to fallback lang bundles and lang entries when needed

This PR introduces a new configuration called `whitelist` to specify an array of required lang bundles. If this value is set, `locator-lang` will complete those lang bundles and/or entries in each bundle based on the `defaultLang`. Here is an example:

```
var Locator = require('locator'),
    LocatorLang = require('locator-lang'),
    loc = new Locator();

// using locator-lang plugin
loc.plug(new LocatorLang({
    defaultLang: 'en',
    whitelist: ['en', 'fr', 'es']
}));
```

* [x] tests
* [x] docs