Add support for nested objects

[cavic19]

Sometimes it is useful to structure a language package object in a nested manner. Such as

//root
{
  hello: "Hi",
  formal: {
    hello: "Hello, sir",
    small_talk: "What a beautiful weather"
  }
}

//zh-hk
{
  hello: "雷吼",
  formal: {
    hello: "你好，先生",
  }
}

Fallback mechanism, however, doesn't work for such a nested structure resulting in an undefined value.

lang.init('zh-hk');
lang.formal.small_talk; //results in undefined

This PR solves this issues by introducing recursive function, that is responsible for the fallback.

[TooBug]

Released in 0.9.4. Thanks for your contribution!