Only ask user to confirm deletion when input is piped
Only ask user to confirm deletion when input is piped

I don't want to be asked "are you sure" in response to:

```
$ stack-delete example-stack
```

but do when I use:

```
$ stacks mike | stack-terminate
```

I'd prefer to see the list of stacks that are to be deleted in
case I use a crummy search term.

azrupani/bash-my-aws,jonhiggs/bash-my-aws,realestate-com-au/bash-my-aws