This changeset allows to have shared templates in a static folder
"templates" present in each luet tree. If the directory is present, it
gets scanned and templated accordingly on top of each package. This
allows to use such folder to store custom blocks to share between
packages.
This is still experimental and subject to change, this is just a first
pass version to provide the feature. It needs to be refined still as it
would be more elegant to use the helm engine properly and map our
structure to the engine instead of adapting it roughly.
Fixes#224
