feature: exclude items based on frontmatter

[davidsneighbour]

Many SSG use a draft field in frontmatter to hide content from publishing. I don't see a way to exclude these items currently from the API index. Something like the following maybe:

global:
  exclude_key_bool: draft
  exclude_key: draft

If we talk about GoHugo then the draft field is a boolean. Other systems might introduce some form of publishing vocabulary like status: draft/review/published that would need specific value/enum configuration.