Add Goldmark as the new default markdown handler

This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.

If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:

```toml
[markup]
defaultMarkdownHandler="blackfriday"
```

Fixes #5963
Fixes #1778
Fixes #6355

markup/goldmark/toc.go

@@ -0,0 +1,102 @@
+// 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 goldmark
+
+import (
+	"bytes"
+
+	"github.com/gohugoio/hugo/markup/tableofcontents"
+
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+var (
+	tocResultKey = parser.NewContextKey()
+	tocEnableKey = parser.NewContextKey()
+)
+
+type tocTransformer struct {
+}
+
+func (t *tocTransformer) Transform(n *ast.Document, reader text.Reader, pc parser.Context) {
+	if b, ok := pc.Get(tocEnableKey).(bool); !ok || !b {
+		return
+	}
+
+	var (
+		toc         tableofcontents.Root
+		header      tableofcontents.Header
+		level       int
+		row         = -1
+		inHeading   bool
+		headingText bytes.Buffer
+	)
+
+	ast.Walk(n, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		s := ast.WalkStatus(ast.WalkContinue)
+		if n.Kind() == ast.KindHeading {
+			if inHeading && !entering {
+				header.Text = headingText.String()
+				headingText.Reset()
+				toc.AddAt(header, row, level-1)
+				header = tableofcontents.Header{}
+				inHeading = false
+				return s, nil
+			}
+
+			inHeading = true
+		}
+
+		if !(inHeading && entering) {
+			return s, nil
+		}
+
+		switch n.Kind() {
+		case ast.KindHeading:
+			heading := n.(*ast.Heading)
+			level = heading.Level
+
+			if level == 1 || row == -1 {
+				row++
+			}
+
+			id, found := heading.AttributeString("id")
+			if found {
+				header.ID = string(id.([]byte))
+			}
+		case ast.KindText:
+			textNode := n.(*ast.Text)
+			headingText.Write(textNode.Text(reader.Source()))
+		}
+
+		return s, nil
+	})
+
+	pc.Set(tocResultKey, toc)
+}
+
+type tocExtension struct {
+}
+
+func newTocExtension() goldmark.Extender {
+	return &tocExtension{}
+}
+
+func (e *tocExtension) Extend(m goldmark.Markdown) {
+	m.Parser().AddOptions(parser.WithASTTransformers(util.Prioritized(&tocTransformer{}, 10)))
+}