Start adding local repository support

* Update -zgen-get-clone-dir() and -zgen-get-clone-url() to check if the url is a valid path to a local git checkout, and if so, clone from there instead of trying to clone from github
* Add .zshplugin, .zsh.plugin and finally .sh as fallback filenames to try and load from in a plugin directory
* Add whitespace between clones to make debugging easier