Pipeline configure API too verbose

[Ortuna]

Specifying the pipeline inside the config block can be removed(config.pipeline = Padrino::Pipeline::AssetPack)

Instead of registering Padrino::Pipeline, we should register Padrino::Pipeline::AssetPack or Padrino::Pipeline::Sprockets. This would also allow for fast config failures closer to the users code incase of invalid configuration. Because the pipeline would be known and allowed options for that pipeline can be retrieved. Right now during the config block the pipeline can be anything.