Add dot notation to input only

This pull request adds support for handling nested arrays using Input::only using dot notation. This was reported by @soxyl in #4726.

For example, if the input is ['developer' => ['age' => 25']] you can now use Input::only('developer.age') to get ['developer.age' => 25]. It used to just return null and you could get a nested input value. I believe this was inconsistent as Input::get('developer.age') would work fine and return 25. This may not be the behaviour that is wanted though as it may make more sense to return ['developer' => ['age' => 25]] instead of ['developer.age' => 25]. Open to any opinions about this.