Use Chroma as new default syntax highlighter

If you want to use Pygments, set `pygmentsUseClassic=true` in your site config.

Fixes #3888

commands/genchromastyles.go

@@ -0,0 +1,70 @@
+// Copyright 2017-present 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 commands
+
+import (
+	"os"
+
+	"github.com/alecthomas/chroma"
+	"github.com/alecthomas/chroma/formatters/html"
+	"github.com/alecthomas/chroma/styles"
+	"github.com/spf13/cobra"
+)
+
+type genChromaStyles struct {
+	style          string
+	highlightStyle string
+	linesStyle     string
+	cmd            *cobra.Command
+}
+
+// TODO(bep) highlight
+func createGenChromaStyles() *genChromaStyles {
+	g := &genChromaStyles{
+		cmd: &cobra.Command{
+			Use:   "chromastyles",
+			Short: "Generate CSS stylesheet for the Chroma code highlighter",
+			Long: `Generate CSS stylesheet for the Chroma code highlighter for a given style. This stylesheet is needed if pygmentsUseClasses is enabled in config.
+
+See https://help.farbox.com/pygments.html for preview of available styles`,
+		},
+	}
+
+	g.cmd.RunE = func(cmd *cobra.Command, args []string) error {
+		return g.generate()
+	}
+
+	g.cmd.PersistentFlags().StringVar(&g.style, "style", "friendly", "highlighter style (see https://help.farbox.com/pygments.html)")
+	g.cmd.PersistentFlags().StringVar(&g.highlightStyle, "highlightStyle", "bg:#ffffcc", "style used for highlighting lines (see https://github.com/alecthomas/chroma)")
+	g.cmd.PersistentFlags().StringVar(&g.linesStyle, "linesStyle", "", "style used for line numbers (see https://github.com/alecthomas/chroma)")
+
+	return g
+}
+
+func (g *genChromaStyles) generate() error {
+	builder := styles.Get(g.style).Builder()
+	if g.highlightStyle != "" {
+		builder.Add(chroma.LineHighlight, g.highlightStyle)
+	}
+	if g.linesStyle != "" {
+		builder.Add(chroma.LineNumbers, g.linesStyle)
+	}
+	style, err := builder.Build()
+	if err != nil {
+		return err
+	}
+	formatter := html.New(html.WithClasses())
+	formatter.WriteCSS(os.Stdout, style)
+	return nil
+}