This commit is multi-fold as it also refactors internally context and logger
as interfaces so it is easier to plug luet as a library externally.
Introduces a garbage collector (related to #227) but doesn't handle yet
parallelism.
Closes#265
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.
Adds a new cli flag to luet build `--pull-repository` which allows to
pass-by a list of docker image references which are used to pull the
cache from
Fixes#185Fixes#184Closes#161
