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