Allow the date/time format on the single template to be overridden

Example:

```toml
[params]
singleDateFormat = '2 January 2006'
```

layouts/_default/single.html

@@ -22,7 +22,7 @@
     <p class="single-readtime">
       {{ with .Date }}
       {{ $dateMachine := . | time.Format "2006-01-02T15:04:05-07:00" }}
-      {{ $dateHuman := . | time.Format ":date_long" }}
+      {{ $dateHuman := . | time.Format (default ":date_long" $.Site.Params.singleDateFormat) }}
       <time datetime="{{ $dateMachine }}">{{ $dateHuman }}</time>
       {{end}}
 
@@ -88,4 +88,4 @@
 
 </div>
 
-{{ end }}
+{{ end }}