Add unique disqus_identifier

Use an optional `uuid` YAML variable to uniquely identify a page for Disqus. This helps match comments with a post in the event of changing page URL, domain, etc. See http://help.disqus.com/customer/portal/articles/472098-javascript-configuration-variables#disqus_identifier
Generate uuid for posts

Generate UUID for unique identifier
Merge remote-tracking branch 'origin/patch-1' into uuid
Merge remote-tracking branch 'origin/patch-2' into uuid