implement support for local extensions update

[Chilipp]

This PR implements the support for an update of local extensions. Once we remove the path from sys.path in the cleanup step, we should also remove all modules that are in this folder from sys.modules

Important notice: Before we can merge this PR, cruft/cookiecutter-test#7 needs to be merged and the temporary change in test_local_extension_update needs to be switched back to the official cruft/cookiecutter-test repo

• add a new filter in the local extensions cookiecutter-test#7 has been merged
• 928b659 has been reverted

closes #253

[Chilipp]

hmm, no idea why this is failing. When I do it locally, i.e.

cruft create https://github.com/Chilipp/cookiecutter-test --directory dir --checkout extensions -y
cruft update --project-dir test --checkout extensions-update --skip-apply-ask

it sometimes works and sometimes doesn't