Jekyll: Import "published: false" frontmatter as a draft

[markstos]

In addition to having a "_drafts" for folder, Jekyll also allows setting "published: false" in the frontmatter.

This could be used for example to "unpublish" a finished post that has expired in usefulness.

Ref: https://jekyllrb.com/docs/posts/#the-posts-folder

I'll take a look to see if I can submit a PR for this.