It holds necessary state plus additional information relative to the
context which we are being run to (e.g. if we are in a terminal or not).
Besides in the future we can use it also as a contextual logger to
provide more smart logging capabilities.
This also replace the general global configuration instance that
previously was share between the core components.
A new keyword `join` is introduced to generate the parent image. It
takes precedence over a `requires` or a `image` already defined in a
spec.
It will generate all the artifacts from the packages listed and join
them in a single image which will be used as parent for the package
build process.
This is a change which invalidates priorly generated hashes.
Fixes#173
