Jekyll changes to support public drafts

_config-collections.yml

collections:
  drafts:
    output: true

_config-defaults.yml

defaults:
  -
    scope:
      path: ""
      type: drafts
    values:
      layout: draft

draft-layout.html

---
layout: post
meta_robots: noindex
---
{{ content }}

draft.html

---
layout:       page
title:        Drafts
permalink:    /drafts/
meta_robots:  noindex
---

<p>These are draft posts for previewing.</p>

<div class="posts">
  {% assign sorted = (site.drafts | sort: 'date') | reverse %}
  {% for draft in sorted %}
    <div class="post py2">
      <p class="post-meta h5">{{ draft.date | date: site.date_format }}</p>
      <a href="{{ draft.url }}" class="post-link"><h3 class="post-title">{{ draft.title }}</h3></a>
      {% if draft.categories.size > 0 %}
        <span class="post-meta small">
          {% for category in draft.categories %}
            <a href="{{ category }}" class="category">{{ category }}</a>
          {% endfor %}
        </span>
      {% endif %}
    </div>
  {% endfor %}
</div>

head.html

{% if page.meta_robots %}    
<meta name="robots" content="{{ page.meta_robots }}">
{% elsif layout.meta_robots %}
<meta name="robots" content="{{ layout.meta_robots }}">
{% else %}
<meta name="robots" content="all">
{% endif %}

Thanks, this has helped me a lot!
I had to make the following changes though:

1. draft.html line 9: I had to remove the parenthesis as I was getting a Liquid warning
2. I wanted to remove the drafts index link from the main navigation. To do that, I had to override the header.html file under _includes and change it to the following:

    {% if my_page.title and my_page.title != 'Drafts' %}
            <a class="page-link" href="{{ my_page.url | relative_url }}">{{ my_page.title | escape }}</a>
    {% endif %}