Allow reusable field group definitions in config file

**- Do you want to request a *feature* or report a *bug*?**

feature

**- What is the current behavior?**

Reuse of configuration data, such as a group of fields being defined once and used in multiple collections, is dependent on YAML features. These features are difficult to understand and use, and aren't available in simpler formats like JSON.

**- What is the expected behavior?**

Netlify CMS should provide means for defining reusable configuration subsets, like field groups, that are easy to understand and use, and format independent.

This is based on the conversation in https://github.com/netlify/netlify-cms/issues/1186#issuecomment-382003132.