Add file (line/col) info to ref/relref errors

See #5371

common/urls/ref.go

@@ -0,0 +1,22 @@
+// Copyright 2018 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 urls
+
+// RefLinker is implemented by those who support reference linking.
+// args must contain a path, but can also point to the target
+// language or output format.
+type RefLinker interface {
+	Ref(args map[string]interface{}) (string, error)
+	RelRef(args map[string]interface{}) (string, error)
+}