* Remove old commands from help output
* Add message to `asdf update` command
* Explain why `asdf global` and `asdf local` have been removed
* Add reference argument to `git.Repoer.Clone` method
* Update `asdf plugin test` command to install specific ref of plugin if provided
* Update `asdf plugin update` command to run pre and post plugin update hooks, and `post-plugin-update` plugin callback
* Enable `plugin_update_command.bats` tests