Add flag to up/down to remove orphaned containers

Add --remove-orphans to CLI reference docs
Add --remove-orphans to bash completion file
Test orphan warning and remove_orphan option in up

Signed-off-by: Joffrey F <joffrey@docker.com>

docs/reference/up.md

@@ -32,6 +32,8 @@ Options:
     -t, --timeout TIMEOUT      Use this timeout in seconds for container shutdown
                                when attached or when containers are already
                                running. (default: 10)
+    --remove-orphans           Remove containers for services not defined in
+                               the Compose file
 
 ```