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/goldmark_config/config.go

@@ -0,0 +1,74 @@
+// 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_config holds Goldmark related configuration.
+package goldmark_config
+
+// DefaultConfig holds the default Goldmark configuration.
+var Default = Config{
+	Extensions: Extensions{
+		Typographer:    true,
+		Footnote:       true,
+		DefinitionList: true,
+		Table:          true,
+		Strikethrough:  true,
+		Linkify:        true,
+		TaskList:       true,
+	},
+	Renderer: Renderer{
+		Unsafe: false,
+	},
+	Parser: Parser{
+		AutoHeadingID: true,
+		Attribute:     true,
+	},
+}
+
+// Config configures Goldmark.
+type Config struct {
+	Renderer   Renderer
+	Parser     Parser
+	Extensions Extensions
+}
+
+type Extensions struct {
+	Typographer    bool
+	Footnote       bool
+	DefinitionList bool
+
+	// GitHub flavored markdown
+	Table         bool
+	Strikethrough bool
+	Linkify       bool
+	TaskList      bool
+}
+
+type Renderer struct {
+	// Whether softline breaks should be rendered as '<br>'
+	HardWraps bool
+
+	// XHTML instead of HTML5.
+	XHTML bool
+
+	// Allow raw HTML etc.
+	Unsafe bool
+}
+
+type Parser struct {
+	// Enables custom heading ids and
+	// auto generated heading ids.
+	AutoHeadingID bool
+
+	// Enables custom attributes.
+	Attribute bool
+}