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.

Added more magic filenames to load inside plugins to be compatible with plugins written for `oh-my-zsh` and `antigen`. 
* Load from `*.zshplugin` or `*.zsh.plugin` if no higher priority files have been found like `antigen` does.
* If we haven't found one of the higher priority plugin files in a plugin directory, source all the *.sh files like `antigen` does.