allow to configure how `reference` field values resolves to file paths

[stefanprobst]

currently, reference fields expect a path to another document file, relative to contentDirPath. it would be great if we could configure how a reference field value resolves to the relative file path.

in my case, i am using "Netlify CMS", which also has a similar concept of relation fields (and saves to markdown/yaml). however, it will only save the slug as field value, and use the folder and extension of a collection to construct file paths. e.g. it will save "my-tag" as relation field value, to reference a document in "tags/my-tag.yml" (when the tags collection has folder: "tags", extension: "yml" configured)