Add support for Flatten JSON format + Restrict translations to default language

[phcollignon]

Hello,

Update 1 :

Some i18n files use a flatten format : i.e.

{
"user.name": "Doe",
"user.firstname": "John"
}

instead of :

{ "user": 
   { "name": "Doe",
     "firstname": "John"
  }
}

I added support for those files with an option generate-translation.flatten in the settings to select the preferred format.

Update 2 :

Developers are usually not the same team as translators, so I add a option to generate only the i18n configuration for the default language. This is optional through a setting : generate-translation.restrictToDefaultLanguage. If left empty translation is done in all languages.

Feel free to merge,

Regards, Philippe