Proposal to change Heroku organization TypeString to a TypeList

Instead of a simple string value for `organization` (added in #389), use a `TypeList` and allow `Locked` and `Personal` values as well. 

Usage:

```
resource "heroku_app" "web" {
  name         = "catsby-test-app"
  region       = "us"
  organization {
    name = "catsby"
    locked = true
    personal = false
  }
}
```

This won't be backwards compatible though, unless you know a trick for supporting both :/