[Proposal] Alternative way for easier custom rule definition for Validator

I am not sure this makes sense to everyone but I'd like to propose an another option to define a custom rule for `Validator`.

**Example code:**

```php
Validator::make(Input::all(), array('name' => array(
    'custom_rule' => function($attribute, $value, $parameters) {
        return true;
    }
)))
```