FR: Allow "delete" to target a specific release

Just thinking about a new feature if this isn't already thought about

If I do `skaffold run`, and I make a change to one particular area, I wouldn't mind being able to do `skaffold delete -r this-particular-release`. To rebuild&redeploy it, I would just do `skaffold run` again.

### Actual behavior

The actual behaviour at the moment is to delete all the releases in that yaml.

### Information

- Skaffold version: 1.8.0
- Operating system: Mojave

### Why not use `skaffold dev`

I have auto-save enabled, so I can cause a lot of skaffold 'churn'.