Feature Request: modal option

[avidh]

@jpillora , thanks for this amazing library. This is my favorite notification js library so far.

I was wondering how difficult it is to support "modal" behavior. It would be really cool to have "modal" option, which makes the notification behave like modal dialog. This feature can be used when you don't want the user to ignore the notification since making it modal would lock the entire page and the user would have to acknowledge and close the notification in order to get back to the page.

Thanks and cheers!