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

helpers/content.go

@@ -32,7 +32,6 @@ import (
 	bp "github.com/gohugoio/hugo/bufferpool"
 	"github.com/gohugoio/hugo/config"
 	"github.com/spf13/afero"
-	jww "github.com/spf13/jwalterweatherman"
 
 	"strings"
 )
@@ -58,9 +57,6 @@ type ContentSpec struct {
 	BuildExpired bool
 	BuildDrafts  bool
 
-	Highlight            func(code, lang, optsStr string) (string, error)
-	defatultPygmentsOpts map[string]string
-
 	Cfg config.Provider
 }
 
@@ -77,36 +73,10 @@ func NewContentSpec(cfg config.Provider, logger *loggers.Logger, contentFs afero
 		Cfg: cfg,
 	}
 
-	// Highlighting setup
-	options, err := parseDefaultPygmentsOpts(cfg)
-	if err != nil {
-		return nil, err
-	}
-	spec.defatultPygmentsOpts = options
-
-	// Use the Pygmentize on path if present
-	useClassic := false
-	h := newHiglighters(spec)
-
-	if cfg.GetBool("pygmentsUseClassic") {
-		if !hasPygments() {
-			jww.WARN.Println("Highlighting with pygmentsUseClassic set requires Pygments to be installed and in the path")
-		} else {
-			useClassic = true
-		}
-	}
-
-	if useClassic {
-		spec.Highlight = h.pygmentsHighlight
-	} else {
-		spec.Highlight = h.chromaHighlight
-	}
-
 	converterProvider, err := markup.NewConverterProvider(converter.ProviderConfig{
 		Cfg:       cfg,
 		ContentFs: contentFs,
 		Logger:    logger,
-		Highlight: spec.Highlight,
 	})
 	if err != nil {
 		return nil, err
@@ -220,6 +190,21 @@ func (c *ContentSpec) RenderMarkdown(src []byte) ([]byte, error) {
 	return b.Bytes(), nil
 }
 
+func (c *ContentSpec) ResolveMarkup(in string) string {
+	in = strings.ToLower(in)
+	switch in {
+	case "md", "markdown", "mdown":
+		return "markdown"
+	case "html", "htm":
+		return "html"
+	default:
+		if conv := c.Converters.Get(in); conv != nil {
+			return conv.Name()
+		}
+	}
+	return ""
+}
+
 // TotalWords counts instance of one or more consecutive white space
 // characters, as defined by unicode.IsSpace, in s.
 // This is a cheaper way of word counting than the obvious len(strings.Fields(s)).