Add an option to specifiy notification container

[eekboom]

If the notification is done on some element that is in a container which has overflow-hidden, then probably only part of the notification will be visible.

I propose an option to specify a container for the notification (which could be the document's body). The position of the notification would need to be adjusted of course, so that it is still graphically positioned correctly.

BTW: Thanks for the work! notifyjs does a nice job for our application.

[EvanCarroll]

@eekboom can you provide a Pull Request for review?