Add render template hooks for links and images

This commit also

* revises the change detection for templates used by content files in server mode.
* Adds a Page.RenderString method

Fixes #6545
Fixes #4663
Closes #6043

markup/converter/hooks/hooks.go

@@ -0,0 +1,57 @@
+// Copyright 2019 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 hooks
+
+import (
+	"io"
+
+	"github.com/gohugoio/hugo/identity"
+)
+
+type LinkContext interface {
+	Page() interface{}
+	Destination() string
+	Title() string
+	Text() string
+}
+
+type Render struct {
+	LinkRenderer  LinkRenderer
+	ImageRenderer LinkRenderer
+}
+
+func (r *Render) Eq(other interface{}) bool {
+	ro, ok := other.(*Render)
+	if !ok {
+		return false
+	}
+	if r == nil || ro == nil {
+		return r == nil
+	}
+
+	if r.ImageRenderer.GetIdentity() != ro.ImageRenderer.GetIdentity() {
+		return false
+	}
+
+	if r.LinkRenderer.GetIdentity() != ro.LinkRenderer.GetIdentity() {
+		return false
+	}
+
+	return true
+}
+
+type LinkRenderer interface {
+	Render(w io.Writer, ctx LinkContext) error
+	identity.Provider
+}