Helper method to check if a view block exists or not

Handy helper method to check if a block has been created.

Useful for if you are using a block inside a html element that cannot be empty and no default make sense:
```php
<?php if($this->exists('table-footer')) :?>
	<tfoot>
		<?=$this->fetch('table-footer');?>
	</tfoot>
<?php endif;?>
```