Make a nice top/bottom center global position

The example is in 'examples/position.html'

examples/position.html

@@ -36,6 +36,13 @@
       });
 
     });
+
+    $.notify('Line text 1', {position: 'top center'});
+    $.notify('Line text 01', {position: 'top center'});
+    $.notify('Line text 001', {position: 'top center'});
+    $.notify('Line text 1', {position: 'bottom center'});
+    $.notify('Line text 01', {position: 'bottom center'});
+    $.notify('Line text 001', {position: 'bottom center'});
   </script>
 
 </body>