Reorganized config? Breaking change - 3.0?

Adding the `shell-integration` in #123 top level key adds to the confusion of reserved keywords at the top level of the config. Additionally, it'd be neat if different sub-commands could have different default backends. For example `ntfy shell-integration` could use local notifications and `nfty done` could use push notifications.

Rough idea...
```yaml
default-backends:
done: pushover
shell-integration: default
shell-integration:
- timeout: 5
backend: default
- timeout: 60
backend: pushover
backends:
pushover:
user_key: hunter2
```