Allow apps to overwrite translation domains from third party plugins.

In 3.x the limitation here is resolved by [this](http://book.cakephp.org/3.0/en/core-libraries/internationalization-and-localization.html#creating-your-own-translators) and [this](http://book.cakephp.org/3.0/en/core-libraries/internationalization-and-localization.html#creating-generic-translators).

But until then we have a huge problem when using third party plugins and i18n.
The plugin domain gets prioritized for `pluginname==domainname` which makes it impossible to overwrite with own translations. This can be especially problematic with languages like German, where there can be multiple forms of translations, depending on whether you address formal or informal.

In order to bridge this until 3.x I suggest a basic configure param.

It is fully BC and non intrusive.