Enable check nested key with '.'

[muddydixon]

This change enables to check the key with "." from nested object. Thus we can check bellow:

var target = {info: {email: "joe@gmail.com"}};
new Checkit({
  "info.email": ["required", "email"]
}).run(target); // => ok

[tgriesser]

Neat, thanks

[muddydixon]

thx!