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

This is a feature request

- What is the current behavior?
Currently placeholders are not supported at field level. That is we can have a slug something like below at collection level but it doesn't work if used at field level.

slug: "{{year}}-{{month}}-{{day}}-{{slug}}"

- What is the expected behavior?
For example having a Field definition:

- {label: "Slug", name: "slug", widget: "hidden", default: "{{slug}}"}

Should produce an entry in the actual document body
Expected outcome in the body for an article titled "Netlify CMS is awesome" should be

slug: netlify-cms-is-awesome