Evaluate plugin considerations

[nerdstein]

To make this tool flexible, I want the project to ship with a base "provisioner" and be extended discretionally thereafter.

The extensions ideally will be managed through Composer, in a similar way to how Robo extensions work. And, most importantly, all extensions should not reinvent the wheel. We should make use of existing Composer and Robo plugins and only write glue code that associates the existing plugins to the outcomes of Bild provisioning.

Links to review:
http://robo.li/extending/
https://github.com/boedah/robo-drush