Allow override of date format

Previously the default date format (`January 2, 2006` ) was used for
blog and single page date visualizations.

The new parameter `date_format` can be used to override this format.
This allows the visualization of localized dates or a changed precision
(down to the minute or up to the month or year of publication).

README.md

 ```
 
 
+### Localize date format
+
+Dates of blog posts and single pages are rendered with the default date format commonly used in the USA and Canada. It is possible to specify a different format.
+
+```
+[params]
+  date_format = "2. January 2006"
+```
+
+See hugo's documentation of the [`dateFormat` function](https://gohugo.io/functions/dateformat/) for more details.
+
+
 ### Nearly finished
 
 In order to see your site in action, run Hugo's built-in local server.

layouts/_default/single.html

       {{ end }}
       {{/* Hugo uses Go's date formatting is set by example. Here are two formats */}}
       <time class="f6 mv4 dib tracked" datetime="{{ .Date.Format "2006-01-02T15:04:05Z07:00" }}">
-        {{- .Date.Format "January 2, 2006" -}}
+        {{- .Date.Format (default "January 2, 2006" .Site.Params.date_format) -}}
       </time>
 
       {{/*

layouts/post/summary.html

   <div class="mb3 pa4 mid-gray overflow-hidden">
     {{ if .Date }}
       <div class="f6">
-        {{ .Date.Format "January 2, 2006" }}
+        {{ .Date.Format (default "January 2, 2006" .Site.Params.date_format) }}
       </div>
     {{ end }}
     <h1 class="f3 near-black">