Settings

Spike on a `settings` command. This is very similar to `labs`, but only lists features in the "general" state. There are currently no features in the "general" state so this isn't useful on it's own.

```
$ bin/heroku settings
=== App Settings (no app specified)
[ ] preboot              Provide seamless web dyno deploys

$ bin/heroku settings --app noah
=== App Settings (noah)
[ ] preboot              Provide seamless web dyno deploys

$ bin/heroku settings:info preboot
=== preboot
Docs:    https://devcenter.heroku.com/articles/labs-preboot
Summary: Provide seamless web dyno deploys

$ bin/heroku settings:enable preboot
 !    Must specify an app

$ bin/heroku settings:enable preboot --app noah
Enabling preboot for noah... failed
 !    Only Cedar apps are supported.

$ bin/heroku settings:enable preboot --app protected-peak-7524
Enabling preboot for protected-peak-7524... done
For more information see: https://devcenter.heroku.com/articles/labs-preboot

$ bin/heroku settings:disable preboot --app protected-peak-7524
Disabling preboot for protected-peak-7524... done
```