Allow 3rd party to alter the config

[claudiu-cristea]

It's more than likely that we'll have introduce tons of dynamic commands and configurations inside runner.yml.dist for https://github.com/ec-europa/joinup-dev project. This will make the file too big and would make it unreadable and hard to structure. We need a way to split dynamic and settings in multiple .yml files.

A mechanism that allows a subscriber to alter the configuration would be helpful.