Modularize config.yml

**Is your feature request related to a problem? Please describe.**

I'm always frustrating when, by defining even a relatively small number of collections, I ended up with a gigantic `config.yml` which is not manageable at all.

**Describe the solution you'd like**

I'd like to be able to break down `config.yml` in small pieces (i.e. yml files).

Those yml files could live in a `config` subdir of `static/admin` directory. Then NetlifyCMS should be able to merge them all together no matter how deep is the `config` tree.

**Describe alternatives you've considered**

The alternative would be to add a step in the build process to create a gigantic yml file composed of the smaller ones.

**Additional context**

This could be very helpful in migrating data intensive website with a lot of collections (i.e. DB table like structure and relationship).