Add configurable timestamping for posts (#494)

* Add configurable timestamping

* Add attribution for configurable timestamping

* Add new post timestamp options to example hugo.toml

* All date formats are now set by single config option in site or page config

* Add post timestamp explainer and examples to example config file

* Add configurable timestamping value to frontmatter archetype

* Explain possible timezone rendering issue with new time formatting

* Fix bug concerning time/date tokens

* Convert datestamps to work with a partial

* Fix issue with lastmod being equal to date

* Add Updated prefix functionality

layouts/_default/list.html

@@ -13,7 +13,7 @@
         <div class="post-meta">
           {{- if .Date -}}
             <time class="post-date">
-              {{- .Date.Format "2006-01-02" -}}
+              {{- partial "post-date" . -}}
             </time>
           {{- end -}}
           {{- with .Params.Author -}}