A new parameter is added to `_autoenv_hash_pair` to specify the version, defaulting to the latest (2). It outputs a `cksum`-based hash for version 2 and `shasum`-based for version 1.
Moves logic to check for an entry in `$AUTOENV_AUTH_FILE` into its own function (`_autoenv_authorized_pair`), as it may need to be called twice.
Modifies `_autoenv_authorized_env_file` to check for v1 entries when v2 fails.
Fixes#53. Alternative implementation to #54.
