Add option to run pre/post commands for any container (#73)

* Add option to run pre commands on arbitrary container

* Correctly handle quoted args in commands

* Provide defaults for test version arg

* Allow filtering of target containers

* Add documentation on exec commands

* Use mysqldump in exec test

* Add mysqldump section to recipes

* Also run commands test in swarm mode

* Use name instead of id

* Add syntax highlighting

* Add missing license headers

test/notifications/docker-compose.yml

@@ -1,8 +1,8 @@
 version: '3'
 
 services:
-  backup: &default_backup_service
-    image: offen/docker-volume-backup:${TEST_VERSION}
+  backup:
+    image: offen/docker-volume-backup:${TEST_VERSION:-canary}
     restart: always
     environment:
       BACKUP_FILENAME: test.tar.gz