[FacebookBridge] Add option to limit number of returned items

This commit adds a new optional parameter 'limit' which can be used
to limit the number of items returned by this bridge (i.e. '&limit=10')

As requested in #669

bridges/FacebookBridge.php

@@ -41,6 +41,15 @@ class FacebookBridge extends BridgeAbstract {
 				'exampleValue' => 'https://www.facebook.com/groups/743149642484225',
 				'title' => 'Insert group name or facebook group URL'
 			)
+		),
+		'global' => array(
+			'limit' => array(
+				'name' => 'Limit',
+				'type' => 'number',
+				'required' => false,
+				'title' => 'Specify the number of items to return (default: -1)',
+				'defaultValue' => -1
+			)
 		)
 	);
 
@@ -117,6 +126,12 @@ class FacebookBridge extends BridgeAbstract {
 
 		}
 
+		$limit = $this->getInput('limit') ?: -1;
+
+		if($limit > 0 && count($this->items) > $limit) {
+			$this->items = array_slice($this->items, 0, $limit);
+		}
+
 	}
 
 	#region Group