hugolib: Add optional outputFormat to Ref/RelRef

Fixes #3224

hugolib/permalinker.go

@@ -0,0 +1,25 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+var (
+	_ Permalinker = (*Page)(nil)
+	_ Permalinker = (*OutputFormat)(nil)
+)
+
+// Permalinker provides permalinks of both the relative and absolute kind.
+type Permalinker interface {
+	Permalink() string
+	RelPermalink() string
+}