Create an `apps` core plugin.

This creates the following commands:

- apps
- apps:create <app>
- apps:destroy <app>

It also:

- makes `delete` an alias for `apps:destroy`
- adds confirmation to `apps:destroy`
- allows a developer to remove apps that have been created via apps:create but have not been deployed

Refs #87
Refs #543
Refs #586
Closes #599
Refs #655
Refs #656
Refs #685
Closes #757
Merge pull request #759 from progrium/apps-plugin

Create an `apps` core plugin.