Feature request to throttle notification sending

[sts-ryan-holton]

Hi, I've recently installed the newest version of this package into my project and have built a feature around allowing a user to set their notification preferrences, such as when they should receive notifications, things like day of week and time of day, this all works great.

If a user has reduced their preferences, notifications for the user will build up and then all send at once at the next available datetime. Would it be possible to introduce a new feature for throttling the sending? For example, add a 10 second difference between each send_at so that users receive bulk notifications 10 seconds apart.

[atymic]

Feel free to PR