Support resource names instead of IDs in more places

Hi,

As I am trying to create my own DigitalOcean provisioning automation using doctl, I find myself having to create more complicated scripts due to the fact that some (most?) of the doctl commands - when they refer to a specific resource - expect ID instead of the friendly name I have given it.

Some commands are friendly and know how to handle names, like this one:

```shell
# doctl compute droplet tag
$ doctl compute droplet tag sandbox --tag-name development
```

While other commands work only with ID, like this one:

```shell
# doctl compute firewall add-droplets --droplet-ids
$ doctl compute firewall add-droplets d9...28 --droplet-ids 1234
```

and it would make life so much easier if we could use names here, like:

```shell
$ doctl compute firewall add-droplets my-default-firewall --droplets sandbox,development
```

Any chance this will be supported?