Enhancement: Allow published field instead of draft fields

[estruyf]

Add the ability to invert the draft field as this allows SSGs to use published fields instead.

[estruyf]

[estruyf]

The frontMatter.content.draftField setting will have a new property called invert which allows you to invert the logic. That way, you can make use of a published field instead of the standard draft field.

{
  "frontMatter.content.draftField": {
    "name": "published",
    "type": "boolean",
    "invert": true
  }
}