Add ability to put new notifications at top or bottom of stack (issue #223)

[MichaelPGriffin]

This introduces functionality to display any new notification messages above or below the currently visible notifications. This behavior is driven from a stackNotifications property on the options object.

Fixes #223

[mansona]

@MichaelPGriffin would it be ok for you to just override the display: flex and the flex-direction: column-reverse of the notification container? 🤔

This sort of thing has not been possible in the past because we are using CSS modules but #251 (comment) is due to change that. Let me know if this solves your problem 👍

Also I would be reluctant to add more config to this addon if we are able to solve the problem another way already 💪

[MichaelPGriffin]

@mansona I hadn't considered a CSS-based approach, thanks for the idea. Offhand I'm not sure if that would change the actual ordering, or just the direction of the stacking.

For my purposes I ended up extending the notifications service and overriding the addNotification() method so the most recently added item would immediately get popped off the end and re-inserted at the beginning.

Also I'm sympathetic your concern about adding more config to this. I'm happy to close this issue as a result.