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 myapp
heroku-postgresql-copper   heroku-postgresql:ronin  Dec  3 20:33
heroku-postgresql-crimson  heroku-postgresql:crane  Nov 19 12:40
papertrail                 papertrail:choklad       Apr 16 15:41
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. Also, the migration on regular resources is still running, which is why the above heroku-postgresql addons are unnamed. They will get names that correspond with their URL names (i.e. `heroku-postgresql-crimson` and `heroku-postgresql-copper`).

Closes #29.

/cc @max