convert nested objects recursively when options.key isn't set

In our system, we use [i18next](http://i18next.com) on the frontend to support i18n. Our translation file contains a lot nested objects, such as:

    "comment": {
        "main_placeholder": "Be the first to comment on this post!",
        "sub_placeholder": "Reply to __original_author__'s comment..."
    }

Currently grunt-pseudolocalize will ignore the nested object, and I modified that by recursively process the nested objects.