Install the plugin if it doesn't exist yet

There should be a way to install a plugin if it doesn't exist. Ideally ensure = enabled would install it too. For example, this is how packages work. There's currently no WP-CLI command to check if a plugin is installed so the `unless` isn't great here. I'm going to try to submit a pull request for `plugin is-installed` to https://github.com/wp-cli/wp-cli/issues/627 some time this week. Other than the `unless`, I think this is pretty good.