resolve heroku-postgresql options to URLs

The Heroku Postgres addon options `fork`, `follow`, and `rollback` need to be resolved from a color/addon name into a color. This PR does that.

```bash
$ hk addon-add heroku-postgresql:crane fork=silver
Added heroku-postgresql:crane to omgwtfbbqapp as heroku-postgresql-bronze.
```

Still filling out the tests around this.