allow custom date formats

This adds a new datef function instead of changing date function in an
effort to keep backwards compatibility.

datef is intentionally used in the example to make it clear to the user
that it is possible to set a custom date format.

in.html

@@ -10,7 +10,7 @@
       <small class="source">
         via <a href="{{.SourceLink}}">{{.SourceTitle}}</a>
       </small>
-      <small class="date">{{.Date | date}}</small>
+      <small class="date">{{.Date | datef "January 2, 2006"}}</small>
     </div>
     {{end}}
   </section>

openring.go

@@ -76,6 +76,9 @@ func main() {
 			"date": func(t time.Time) string {
 				return t.Format("January 2, 2006")
 			},
+			"datef": func(fmt string, t time.Time) string {
+				return t.Format(fmt)
+			},
 		}).
 		Parse(string(input))
 	if err != nil {