Added Pushover as a notification alternative!

[NoxRare]

Description

I've added support for Pushover as a notification service.

Type of change

Notification Service addition.

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

I've been testing built my own Docker image using these changes for well over a month with no issues.

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.
I've only ran this using docker on my Raspberry Pi and have had no issues with the code.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md)
• My changes generate no new warnings
• Any dependent changes have been updated in requirements.txt