[WIP] Create route in `VariantController`

Fixes #44

Add to the controller the chance to generate the route with a closure instead of a named route.
This is due to `getResponse` not having the choice to add parameters to the route.

I don't really like this change, maybe we should consider refactoring the helpers in `AbstractAdminController`…
I don't really like this one, but it's the most generic solution I could think of.
@alch suggested to add a new parameter or make an array detection.
Ping @mmoreram ?