markup/goldmark: Make auto IDs GitHub compatible

You can turn off this behaviour:

```toml
[markup]
  [markup.goldmark]
    [markup.goldmark.parser]
      autoHeadingIDAsciiOnly = true
```
Note that the `anchorize` now adapts its behaviour depending on the default Markdown handler.

Fixes #6616

markup/goldmark/autoid_test.go

@@ -0,0 +1,121 @@
+// 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
+
+import (
+	"strings"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestSanitizeAnchorName(t *testing.T) {
+	c := qt.New(t)
+
+	// Tests generated manually on github.com
+	tests := `
+God is good: 神真美好
+Number 32
+Question?
+1+2=3
+Special !"#$%&(parens)=?´* chars
+Resumé
+One-Hyphen
+Multiple--Hyphens
+Trailing hyphen-
+Many   spaces  here
+Forward/slash
+Backward\slash
+Under_score
+`
+
+	expect := `
+god-is-good-神真美好
+number-32
+question
+123
+special-parens-chars
+resumé
+one-hyphen
+multiple--hyphens
+trailing-hyphen-
+many---spaces--here
+forwardslash
+backwardslash
+under_score
+`
+
+	tests, expect = strings.TrimSpace(tests), strings.TrimSpace(expect)
+
+	testlines, expectlines := strings.Split(tests, "\n"), strings.Split(expect, "\n")
+
+	if len(testlines) != len(expectlines) {
+		panic("test setup failed")
+	}
+
+	for i, input := range testlines {
+		input := input
+		expect := expectlines[i]
+		c.Run(input, func(c *qt.C) {
+			b := []byte(input)
+			got := string(sanitizeAnchorName(b, false))
+			c.Assert(got, qt.Equals, expect)
+			c.Assert(sanitizeAnchorNameString(input, false), qt.Equals, expect)
+			c.Assert(string(b), qt.Equals, input)
+		})
+	}
+}
+
+func TestSanitizeAnchorNameAsciiOnly(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(sanitizeAnchorNameString("god is神真美好 good", true), qt.Equals, "god-is-good")
+	c.Assert(sanitizeAnchorNameString("Resumé", true), qt.Equals, "resume")
+
+}
+
+func BenchmarkSanitizeAnchorName(b *testing.B) {
+	input := []byte("God is good: 神真美好")
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		result := sanitizeAnchorName(input, false)
+		if len(result) != 24 {
+			b.Fatalf("got %d", len(result))
+
+		}
+	}
+}
+
+func BenchmarkSanitizeAnchorNameAsciiOnly(b *testing.B) {
+	input := []byte("God is good: 神真美好")
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		result := sanitizeAnchorName(input, true)
+		if len(result) != 12 {
+			b.Fatalf("got %d", len(result))
+
+		}
+	}
+}
+
+func BenchmarkSanitizeAnchorNameString(b *testing.B) {
+	input := "God is good: 神真美好"
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		result := sanitizeAnchorNameString(input, false)
+		if len(result) != 24 {
+			b.Fatalf("got %d", len(result))
+		}
+	}
+}