ability to limit results via filters and excludes

example usage 

```php
$grid = new GridField(
	'ExampleGrid',
	'Example Grid',
	$this->Items(),
	GridFieldConfig::create()
		->addComponent(new GridFieldButtonRow('before'))
		->addComponent(new GridFieldToolbarHeader())
		->addComponent(new GridFieldDeleteAction(true))
		->addComponent($component = new GridFieldAddExistingSearchButton())
);

// add filters to the GridFieldAddExistingSearchButton component
$component->setSearchFilters(array('Status' => 'active));

// add excludes
$component->setSearchExcludes(array('ID' => 45));
```