Stable repositories for ready to use app specification

**What is the problem this feature would solve? Please describe.**
Facilitates deploying ready to use applications (e.g. Wordpress, Ghost, Meilisearch, etc) with just a minimal configuration

**Describe the solution you'd like**
Support of as part of the `doctl apps` subcommands

**Additional context**

If this is not something that is considered in the near future, then I have a proposal for this as part of the `appfile` tool at https://github.com/renehernandez/appfile/issues/43. I would love feedback on this.