brewkit.rb changes ENV destructively, so lets not do that everytime a formula
is required. Now it's possible for other tools to require a formula
description without worrying about side-effects.
Pass in a list of any files that you don't want cleaned
with a path relative to the cellar. e.g. `strip_paths ['bin/znc']`
It's backwards compatible with def strip_clean?, at least for now.
The znc formula is updated as an example.
