Support phrases in nested JSONs through the t() method

I migrated to polyglot from another library that allowed nested JSONs as such:

```js
var polyglot = new Polyglot({
  phrases: {
    "hello": "Hello",
    "actions": {
      "delete": "Delete",
      "add": "Add"
    }
  }
});

polyglot.t('actions.add') // "Add"
```

This wasn't possible with Polyglot so I added it. Simply add allowNestedJSON: true when instantiating Polyglot, and it'll work.

I know there's a similar PR opened, but I think it's cleaner to have everything pass through the t() method.

Let me know what you think.