Multiple Configuration Files

NetlifyCMS reads several configuration files.

Extension 1: If several configuration files set the same parameter, the configuration will be overwritten in a defined order. For example, the order in which the configuration files are named could also define the priority.

Extension 2: Tree structures like the collections are merged intelligently. With this it would be possible to add or remove Collections through another configuration file. Similarly, individual fields could be added, removed or changed in their type.

Extension 3: With many configuration files or complex changes, it is difficult for users to infer the merged and finally used configuration. Therefore NetlifyCMS offers the possibility to display the merged configuration. Ideally NetlifyCMS will show from which file a parameterization is taken.

**Is your feature request related to a problem? Please describe.**
I would like to provide users with a theme (in this case for Hugo). This theme should already contain a NetlifyCMS configuration. The user has to adjust some local parameters (e.g. `backend`). I want to customize and develop the theme and the NetlifyCMS configuration centrally. Changes in the configuration should be visible in the user's instance after deploy with a new theme version without further adaptations.

**Describe the solution you'd like**
Multiple `link` entries in the header
```

```

**Describe alternatives you've considered**
I considered a manual initialization (https://www.netlifycms.org/docs/beta-features/#manual-initialization). However I would prefere a more generic no-code solution.

**Additional context**