Update Desktop Content

Add the ability to update existing desktop notifications.

````javascript
var note = new PNotify({
					title:"Alpha",
					text:"Alpha is the first letter",
					desktop:{
						desktop:true,
						tag:"Alpha"
					}
				});
note.update({
		title:"Bravo",
		text:"Bravo comes next in line"
	});
````

By adding the tag option to the desktop object, existing notifications can be updated.

[Using Web Notifications](https://developer.mozilla.org/en-US/docs/WebAPI/Using_Web_Notifications#Dealing_with_repeated_notifications)