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

Add [a `logging.Handler` sub-class](https://docs.python.org/3/howto/logging.html#handlers) 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.