Add local repo support to zgen load

If the argument to `zgen load` is a path to a valid local git repository (The check is not sophisticated, it just looks for a .git directory), instead of trying to clone it from github, we do a local clone from that path. This is useful in at least the following scenarios:
* **Private repositories:** You may have a private plugin that includes confidential data and is not published on github, even as a private repository.
* **Plugin development:** You can work on a plugin without having to push a stream of intermittent commits to github that may break someone's system.