Allow to pass additional arguments.

I would like to be able to pass additional arguments to notify-send, for example to control the timeout, or to specify a different icon, and to be able to pass a title as well as a message to the notification.

```
/set plugins.var.perl.notify_send.command "notify-send --expire-time=60000 --icon=network $name $message"
```

This pull request still splits the string on space boundaries, so it only supports long arguments (eg '--expire-time=60000' rather than '-t 60000').