Multiple timeouts for shell-integration

Instead of only using the default(per config) backend and one timeout, the shell-integration could support various timeouts with different backends each. For example, local notification if longer than 5 seconds, push notification if longer than a minute.

Config idea:
```yaml
shell-integration:
- timeout: 5
backend: default
- timeout: 60
backend: pushover
```

makes the toplevel `shell-integration` key special (like `backend` and `backends`)
Should consider sections? see #124 for more ideas.