Several improvements and fixes

* Fix issues with `rbenv plugin list` `rbenv plugin update all`, `rvbenv plugin delete all` on MacOS
* Make installation of [approved plugins](https://github.com/sstephenson/rbenv/wiki/Plugins#approved-plugins) more easy – instead of `rbenv plugin install tpope:rbenv-ctags` you can type just `rbenv plugin install ctags`
* Some improvements in `rbenv plugin-install`, `rbenv plugin-delete` and `rbenv plugin-update` autocompletion
* `rbenv plugin updata` will update all plugins instead of  showing error message