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/cli/run.sh

@@ -44,7 +44,7 @@ docker run --rm \
   --env BACKUP_FILENAME=test.tar.gz \
   --env "BACKUP_FROM_SNAPSHOT=true" \
   --entrypoint backup \
-  offen/docker-volume-backup:$TEST_VERSION
+  offen/docker-volume-backup:${TEST_VERSION:-canary}
 
 docker run --rm -it \
   -v backup_data:/data alpine \