First pass at a WP-CLI command to install all the plugins

For VIP Quickstart, it would be nice to be able to programmatically install all the recommended plugins with WP-CLI. This isn't super robust, but gets the job done for now. For example, there's a semi-weak check for weather or not the plugin in question is already installed. It would be better if WP-CLI had a way to check whether a plugin was installed without killing the script. Related: https://github.com/wp-cli/wp-cli/issues/627