feat(log): Add a logging handler for use from Python

[rpatterson]

Add a logging.Handler sub-class so that Python projects can re-use a user's ~/.ntfy.yml configuration to send application-specific notifications using its logging configuration. An application could then use its logging configuration to send notifications, for example for all messages of level WARNING or ERROR, or any of the other useful ways logging can be configured.

[rpatterson]

Soon, I need to add notifications to a Python CLI utility. I use and love ntfy for notifications from my shell, so I intend to use it in that project using the approach in the description. I could implement it as a separate distribution/package but if the maintainers are interested in merging an accepted PR adding that to ntfy itself, just LMK and I'll do that instead.