add before_view() method to ViewModel

The ``ViewModel::before()`` method excuted on ``ViewModel::forge()``.  
``before()`` can't get params of set after forge.

```php
example: <?php
//on controller
$this->template->content = ViewModel::forge('foo/bar');
$this->template->content->set('param_a', $param_a);

//on class View_Foo_Bar
public function before()
{
    Debug::dump($this->param_a); // OutOfBoundsException [ Error ]: View variable is not set:param_a
}
```

The ``before_view()`` method can do it.