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

plugins/apps/pre-delete

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+APP="$1"; IMAGE="dokku/$APP"; CACHE_DIR="$DOKKU_ROOT/$APP/cache"
+if [[ -d $CACHE_DIR ]]; then
+  docker run -v "$CACHE_DIR:/cache" "$IMAGE" find /cache -depth -mindepth 1 -maxdepth 1 -exec rm -Rf {} \;
+fi