Support named addons

@brandur just shipped named addons in the v3 API. We'll now use them as the identity for actions involving an addon that's already been created (remove + open).

```bash
➜ ./hk addons -a hkdist
[unnamed]   heroku-postgresql:ronin  Dec  3 20:33
[unnamed]   heroku-postgresql:crane  Nov 19 12:40
papertrail  papertrail:choklad       Aug  8 12:50
pgbackups   pgbackups:plus           Sep 30 15:43
```

Note that attachable resources will still be broken for the next few weeks, as these won't have names populated yet, and the migration on regular resources is still running (which is why the above heroku-postgresql addons are unnamed).

Closes #29.

/cc @max