Adds dokku apps:rename function, closes #1440.

Adds unit test for new apps:rename function.

Add example in the docs for new apps:rename function.

Add apps:rename command to help command.

Delete cache directory before copying old app directory to new app directory.

deploy_app instead of create_app for apps:rename testing.

Change app url to match new name. Fix unit tests.

Add some more tests, plugn triggers.

tests/unit/apps.bats

@@ -26,3 +26,23 @@ load test_helper
   echo "status: "$status
   assert_success
 }
+
+@test "(apps) apps:rename" {
+  deploy_app
+  run bash -c "dokku apps:rename $TEST_APP great-test-name"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+  run bash -c "dokku apps | grep $TEST_APP"
+  echo "output: "$output
+  echo "status: "$status
+  assert_output ""
+  run bash -c "curl --silent --write-out '%{http_code}\n' `dokku url great-test-name` | grep 404"
+  echo "output: "$output
+  echo "status: "$status
+  assert_output ""
+  run bash -c "dokku --force apps:destroy great-test-name"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+}