Add support for other notification types

[MrTyton]

Add support for more than just pushbullet notifications.

See 1 2 for implementation of class and docstrings.

While this does work locally, I have not pushed this live to the docker image in case - we can do this after adding in some more types and more thoroughly testing things. I probably should have created a branch but didn't feel like it.

@ahuston-0

[ahuston-0]

Cool, I'll pull this down to my local instance and do some testing!

[MrTyton]

Check out https://github.com/MrTyton/AutomatedFanfic/tree/discord

I have not tested this