Autoload and custom group for settings

These are two minor commits that:

- Make `logstash-conf-mode` autoloaded so a user can do `M-x logstash-conf-mode` without requiring the package first
- Create a `logstash` group and add the `logstash-indent` setting to it so it can be customized. Any future settings can be added to this group so someone can do `M-x customize-group logstash` and see all the settings.