mergeatron: improve plugin loading

- When the config has no plugin configurations, mergeatron exits gracefully.
- When there is no configuration for a specific plugin, the plugin is not loaded.
- A plugin being enabled implies that there **is** a configuration for it **and** one of two things: `config.enabled` is truthy, or undefined. i.e.: a plugin is disabled when `config.enabled` is explicitly set to `false`

The reason for the first two changes is due to the fact that most of these plugins will simply not run without a configuration.
The third is...just because it kind of makes more sense. The only time when a plugin should be disabled is when `enabled` is `false` or when there is no config at all. The startup logic now more explicitly conveys this.