Document -l flag and exclude librarian from purge

Librarian installs packages to modules directory and caches package information
to .tmp. Excluding both these directories from `git clean` when using
librarian-puppet prevents the script from having to reinstall each package from
scratch every time, which can take a while with a large number of packages.
Merge oskar's changes and restrict mass delete

Conflicts:
	puppet-sync
Recreate .tmp directory