Refactor generate command

[gossi]

$ keeko g(enerate) <blueprint>

Similarly to ember g <blueprint>. However, the keeko command is still a subclass of the symfony command and will benefit from it's interactive console part to ask for information if no enough are provided.

At some point it should be possible to extend the blueprints as they do it in ember, yet there should be some form of "registration" within keeko. Maybe all blueprints should be a composer package with type keeko-blueprint. Additionally, keeko-cli is meant to be installed globally, those blueprints may installed per-project. I have no idea how to handle that.