Add markdownify template filter

Note that this is a Markdownify filter, and is named as such; it's not a Asccidoc filter or in any way connected to a Page.

Fixes #524

tpl/template_test.go

@@ -1,6 +1,7 @@
 package tpl
 
 import (
+	"html/template"
 	"reflect"
 	"testing"
 )
@@ -339,3 +340,14 @@ func TestWhere(t *testing.T) {
 		}
 	}
 }
+
+func TestMarkdownify(t *testing.T) {
+
+	result := Markdownify("Hello **World!**")
+
+	expect := template.HTML("<p>Hello <strong>World!</strong></p>\n")
+
+	if result != expect {
+		t.Errorf("Markdownify: got '%s', expected '%s'", result, expect)
+	}
+}