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/highlight/config.go

@@ -0,0 +1,188 @@
+// 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 highlight provides code highlighting.
+package highlight
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/alecthomas/chroma/formatters/html"
+
+	"github.com/gohugoio/hugo/config"
+
+	"github.com/mitchellh/mapstructure"
+)
+
+var DefaultConfig = Config{
+	// The highlighter style to use.
+	// See https://xyproto.github.io/splash/docs/all.html
+	Style:              "monokai",
+	LineNoStart:        1,
+	CodeFences:         true,
+	NoClasses:          true,
+	LineNumbersInTable: true,
+	TabWidth:           4,
+}
+
+//
+type Config struct {
+	Style string
+
+	CodeFences bool
+
+	// Use inline CSS styles.
+	NoClasses bool
+
+	// When set, line numbers will be printed.
+	LineNos            bool
+	LineNumbersInTable bool
+
+	// Start the line numbers from this value (default is 1).
+	LineNoStart int
+
+	// A space separated list of line numbers, e.g. “3-8 10-20”.
+	Hl_Lines string
+
+	// TabWidth sets the number of characters for a tab. Defaults to 4.
+	TabWidth int
+}
+
+func (cfg Config) ToHTMLOptions() []html.Option {
+	var options = []html.Option{
+		html.TabWidth(cfg.TabWidth),
+		html.WithLineNumbers(cfg.LineNos),
+		html.BaseLineNumber(cfg.LineNoStart),
+		html.LineNumbersInTable(cfg.LineNumbersInTable),
+		html.WithClasses(!cfg.NoClasses),
+	}
+
+	if cfg.Hl_Lines != "" {
+		ranges, err := hlLinesToRanges(cfg.LineNoStart, cfg.Hl_Lines)
+		if err == nil {
+			options = append(options, html.HighlightLines(ranges))
+		}
+	}
+
+	return options
+}
+
+func applyOptionsFromString(opts string, cfg *Config) error {
+	optsm, err := parseOptions(opts)
+	if err != nil {
+		return err
+	}
+	return mapstructure.WeakDecode(optsm, cfg)
+}
+
+// ApplyLegacyConfig applies legacy config from back when we had
+// Pygments.
+func ApplyLegacyConfig(cfg config.Provider, conf *Config) error {
+	if conf.Style == DefaultConfig.Style {
+		if s := cfg.GetString("pygmentsStyle"); s != "" {
+			conf.Style = s
+		}
+	}
+
+	if conf.NoClasses == DefaultConfig.NoClasses && cfg.IsSet("pygmentsUseClasses") {
+		conf.NoClasses = !cfg.GetBool("pygmentsUseClasses")
+	}
+
+	if conf.CodeFences == DefaultConfig.CodeFences && cfg.IsSet("pygmentsCodeFences") {
+		conf.CodeFences = cfg.GetBool("pygmentsCodeFences")
+	}
+
+	if cfg.IsSet("pygmentsOptions") {
+		if err := applyOptionsFromString(cfg.GetString("pygmentsOptions"), conf); err != nil {
+			return err
+		}
+	}
+
+	return nil
+
+}
+
+func parseOptions(in string) (map[string]interface{}, error) {
+	in = strings.Trim(in, " ")
+	opts := make(map[string]interface{})
+
+	if in == "" {
+		return opts, nil
+	}
+
+	for _, v := range strings.Split(in, ",") {
+		keyVal := strings.Split(v, "=")
+		key := strings.ToLower(strings.Trim(keyVal[0], " "))
+		if len(keyVal) != 2 {
+			return opts, fmt.Errorf("invalid Highlight option: %s", key)
+		}
+		if key == "linenos" {
+			opts[key] = keyVal[1] != "false"
+			if keyVal[1] == "table" || keyVal[1] == "inline" {
+				opts["lineNumbersInTable"] = keyVal[1] == "table"
+			}
+		} else {
+			opts[key] = keyVal[1]
+		}
+	}
+
+	return opts, nil
+}
+
+// startLine compansates for https://github.com/alecthomas/chroma/issues/30
+func hlLinesToRanges(startLine int, s string) ([][2]int, error) {
+	var ranges [][2]int
+	s = strings.TrimSpace(s)
+
+	if s == "" {
+		return ranges, nil
+	}
+
+	// Variants:
+	// 1 2 3 4
+	// 1-2 3-4
+	// 1-2 3
+	// 1 3-4
+	// 1    3-4
+	fields := strings.Split(s, " ")
+	for _, field := range fields {
+		field = strings.TrimSpace(field)
+		if field == "" {
+			continue
+		}
+		numbers := strings.Split(field, "-")
+		var r [2]int
+		first, err := strconv.Atoi(numbers[0])
+		if err != nil {
+			return ranges, err
+		}
+		first = first + startLine - 1
+		r[0] = first
+		if len(numbers) > 1 {
+			second, err := strconv.Atoi(numbers[1])
+			if err != nil {
+				return ranges, err
+			}
+			second = second + startLine - 1
+			r[1] = second
+		} else {
+			r[1] = first
+		}
+
+		ranges = append(ranges, r)
+	}
+	return ranges, nil
+
+}