Add ability to protect (or ignore) certain droplets so they can't be demolished inadvertently

**What is the problem this feature would solve? Please describe.**
Sometimes when I'm using doctl, I manage to delete the wrong droplet.

**Describe the solution you'd like**
I'd like to know if it would be possible to add a way to ignore certain droplets so doctl can't demolish them inadvertently.

**Additional context**
It could be added to the existing `$HOME/.config/doctl/config.yaml` or a new file called `$HOME/.config/doctl/protect.yaml`

Example:

```
protect:
droplets: "293857,1230985720643,1031689246,283756198,9237859183,9128375135"
```