[Feature] Add publish state

[carbontwelve]

A post should have the following publish state with "published" being the default:

• draft
• scheduled
• published

This can be done in one of two ways:

• Replace the draft flag with a publishState enum
• Programatically decide state following the following rules:
  • any post with draft: true front matter is a draft
  • else any post with a date in the future is scheduled
  • else any post with a date in the past is published
  • an exception will apply for stub posts as these are both scheduled and published in their current state; all other scheduled posts should be treated as drafts and NOT be published

I think doing so programatically is the solution I will go with, replacing is_draft functionality for is_published. There is a long standing bug where draft content is being published, I need to fix that in closing this issue.