template service: add Render() method

service/template/service.go

@@ -1,6 +1,7 @@
 package template
 
 import (
+	"io"
 	"net/http"
 
 	"github.com/pkg/errors"
@@ -13,6 +14,7 @@ const ServiceName service.Name = "template"
 // Service is a templating service
 type Service interface {
 	RenderPage(w http.ResponseWriter, templateName string, data interface{}) error
+	Render(w io.Writer, templateName string, data interface{}) error
 }
 
 // From retrieves the template service in the given container