Add support for Netlify CMS preview_path

preview_path allows for UI linkage between the Netlify CMS UI
and the associated preview deploy build. Also adds a commented
out publish_mode configuration to enable the UI authoring flow,
which continues to default to publish direct to master.

static/admin/config.yml

@@ -5,12 +5,17 @@ backend:
 media_folder: "static/media"
 public_folder: "/media"
 
+# Uncomment to leverage Netlify CMS UI authoring flow
+# see: https://www.netlifycms.org/docs/configuration-options/#publish-mode
+# publish_mode: editorial_workflow
+
 collections:
   - name: "posts"
     label: "Posts"
     folder: "content/posts"
     create: true
     slug: "{{year}}-{{month}}-{{day}}-{{slug}}"
+    preview_path: "posts/{{fields.slug}}"
     fields:
       - {label: "Template", name: "template", widget: "hidden", default: "post"}
       - {label: "Title", name: "title", widget: "string"}