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_test.go

@@ -0,0 +1,29 @@
+// 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 (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestRemoveAccents(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(string(RemoveAccents([]byte("Resumé"))), qt.Equals, "Resume")
+	c.Assert(string(RemoveAccents([]byte("Hugo Rocks!"))), qt.Equals, "Hugo Rocks!")
+	c.Assert(string(RemoveAccentsString("Resumé")), qt.Equals, "Resume")
+
+}