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

cmd/backup/config.go

@@ -39,4 +39,6 @@ type Config struct {
 	WebdavPath                 string        `split_words:"true" default:"/"`
 	WebdavUsername             string        `split_words:"true"`
 	WebdavPassword             string        `split_words:"true"`
+	ExecLabel                  string        `split_words:"true"`
+	ExecForwardOutput          bool          `split_words:"true"`
 }