Prexecute and postexecute scripts

[SrZorro]

Add the ability to execute scripts before and after volumerize does his job.

In my setup i need to backup a mariadb and mongodb containers, but instead of saving the volumes as it is i need a way to backup the databases processed (aka mongodump and mysqldump) and the final processed result save it with volumerize.

So, instead of adding a special way to do it for mongo and another for mysql, i think its better if there is some way to execute arbitrary scripts, so if another person has a diferent need (like copying some files around or what not) he can implement his own prexecute and postexecute strategies.