feature: make `ntfy pub` use default-user and default-password from client.yml

Since `ntfy pub` uses the `default-host` defined in client.yml, I expected it to use the `default-user` and `default-password` defined there as well. Apparently it only uses the default user/pass for `ntfy sub` right now.

I can probably make the necessary code changes, but I wanted to log the issue here in the meantime