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` or when there is no configuration for it at all.

The reason for the first two changes is due to the fact that most of these plugins will simply not run without a configuration, both of which are the basis of the third point - 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.