Notifications Can Replaced

Hi!

I added a defaults variable 'replace' that will remove the last notification before adding a new one in the update method.

When combined with 'pool', 'sticky', and 'glue: before', i am able to use the plugin to keep the latest 'pool' number of notifications on the screen and have them update from the top.

There may already be a combination of variables that does this, and there may be a more elegant way to do so, but i wanted to share.

Awesome plugin, and thank you for sharing!