Migrations as services

[arendjantetteroo]

In latest version of doctrine orm fixtures, it's now possible to have fixtures as services which provides all the dependency injection goodies and no issues with getting public services from the container.

I've added this to the mongoodm bundle as a pull request (doctrine/DoctrineMongoDBBundle#529).
Would there be interest for this in this migrations bundle as well?

It would mean implementing an interface which is loaded with a compiler pass into the migration manager instead of loading files inside migrations/mongodb directory.