Installing plugins that have already been installed

Currently, if you install a plugin that has already been installed, it will return the following as if it is actually installing something the first time.

```
./plugin install logstash-input-kafka
Validating logstash-input-kafka
Installing logstash-input-kafka
```

Certainly, `./plugin update` is the way to go for updating installed plugins. It may be nice though to have a different message when someone attempts to use `./plugin install` to "update" the plugin. Maybe something along the lines of "plugin_name is already installed. If you are updating the plugin, please use the `./plugin update plugin_name` command instead.