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

common/text/transform.go

@@ -0,0 +1,47 @@
+// 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 text
+
+import (
+	"sync"
+	"unicode"
+
+	"golang.org/x/text/runes"
+	"golang.org/x/text/transform"
+	"golang.org/x/text/unicode/norm"
+)
+
+var accentTransformerPool = &sync.Pool{
+	New: func() interface{} {
+		return transform.Chain(norm.NFD, runes.Remove(runes.In(unicode.Mn)), norm.NFC)
+	},
+}
+
+// RemoveAccents removes all accents from b.
+func RemoveAccents(b []byte) []byte {
+	t := accentTransformerPool.Get().(transform.Transformer)
+	b, _, _ = transform.Bytes(t, b)
+	t.Reset()
+	accentTransformerPool.Put(t)
+	return b
+}
+
+// RemoveAccentsString removes all accents from s.
+func RemoveAccentsString(s string) string {
+	t := accentTransformerPool.Get().(transform.Transformer)
+	s, _, _ = transform.String(t, s)
+	t.Reset()
+	accentTransformerPool.Put(t)
+	return s
+}