Add canvas destination to open URL action

Author: leocavalcante

lib/src/open_url_action.dart

@@ -3,6 +3,9 @@ class OpenUrlAction {
 
   OpenUrlAction(this.url);
 
+  OpenUrlAction.canvas(String canvasId)
+      : url = 'https://fb.com/canvas_doc/$canvasId';
+
   Map<String, dynamic> toMap() {
     return {'url': url};
   }

test/open_url_action_test.dart

@@ -0,0 +1,10 @@
+import 'package:test/test.dart';
+
+import '../lib/src/open_url_action.dart';
+
+void main() {
+  test('OpenUrlAction.canvas', () {
+    final action = new OpenUrlAction.canvas('test');
+    expect(action.url, equals('https://fb.com/canvas_doc/test'));
+  });
+}