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/markup_config/config.go

@@ -0,0 +1,105 @@
+// 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 markup_config
+
+import (
+	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/docshelper"
+	"github.com/gohugoio/hugo/markup/blackfriday/blackfriday_config"
+	"github.com/gohugoio/hugo/markup/goldmark/goldmark_config"
+	"github.com/gohugoio/hugo/markup/highlight"
+	"github.com/gohugoio/hugo/markup/tableofcontents"
+	"github.com/gohugoio/hugo/parser"
+	"github.com/mitchellh/mapstructure"
+)
+
+type Config struct {
+	// Default markdown handler for md/markdown extensions.
+	// Default is "goldmark".
+	// Before Hugo 0.60 this was "blackfriday".
+	DefaultMarkdownHandler string
+
+	Highlight       highlight.Config
+	TableOfContents tableofcontents.Config
+
+	// Content renderers
+	Goldmark    goldmark_config.Config
+	BlackFriday blackfriday_config.Config
+}
+
+func Decode(cfg config.Provider) (conf Config, err error) {
+	conf = Default
+
+	m := cfg.GetStringMap("markup")
+	if m == nil {
+		return
+	}
+
+	err = mapstructure.WeakDecode(m, &conf)
+	if err != nil {
+		return
+	}
+
+	if err = applyLegacyConfig(cfg, &conf); err != nil {
+		return
+	}
+
+	if err = highlight.ApplyLegacyConfig(cfg, &conf.Highlight); err != nil {
+		return
+	}
+
+	return
+}
+
+func applyLegacyConfig(cfg config.Provider, conf *Config) error {
+	if bm := cfg.GetStringMap("blackfriday"); bm != nil {
+		// Legacy top level blackfriday config.
+		err := mapstructure.WeakDecode(bm, &conf.BlackFriday)
+		if err != nil {
+			return err
+		}
+	}
+
+	if conf.BlackFriday.FootnoteAnchorPrefix == "" {
+		conf.BlackFriday.FootnoteAnchorPrefix = cfg.GetString("footnoteAnchorPrefix")
+	}
+
+	if conf.BlackFriday.FootnoteReturnLinkContents == "" {
+		conf.BlackFriday.FootnoteReturnLinkContents = cfg.GetString("footnoteReturnLinkContents")
+	}
+
+	return nil
+
+}
+
+var Default = Config{
+	DefaultMarkdownHandler: "goldmark",
+
+	TableOfContents: tableofcontents.DefaultConfig,
+	Highlight:       highlight.DefaultConfig,
+
+	Goldmark:    goldmark_config.Default,
+	BlackFriday: blackfriday_config.Default,
+}
+
+func init() {
+	docsProvider := func() map[string]interface{} {
+		docs := make(map[string]interface{})
+		docs["markup"] = parser.LowerCaseCamelJSONMarshaller{Value: Default}
+		return docs
+
+	}
+	// TODO(bep) merge maps
+	docshelper.AddDocProvider("config", docsProvider)
+}