Prepare for Goldmark

This commmit prepares for the addition of Goldmark as the new Markdown renderer in Hugo.

This introduces a new `markup` package with some common interfaces and each implementation in its own package.

See #5963

markup/markup.go

@@ -0,0 +1,83 @@
+// 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 markup
+
+import (
+	"strings"
+
+	"github.com/gohugoio/hugo/markup/org"
+
+	"github.com/gohugoio/hugo/markup/asciidoc"
+	"github.com/gohugoio/hugo/markup/blackfriday"
+	"github.com/gohugoio/hugo/markup/converter"
+	"github.com/gohugoio/hugo/markup/mmark"
+	"github.com/gohugoio/hugo/markup/pandoc"
+	"github.com/gohugoio/hugo/markup/rst"
+)
+
+func NewConverterProvider(cfg converter.ProviderConfig) (ConverterProvider, error) {
+	converters := make(map[string]converter.Provider)
+
+	add := func(p converter.NewProvider, aliases ...string) error {
+		c, err := p.New(cfg)
+		if err != nil {
+			return err
+		}
+		addConverter(converters, c, aliases...)
+		return nil
+	}
+
+	if err := add(blackfriday.Provider, "md", "markdown", "blackfriday"); err != nil {
+		return nil, err
+	}
+	if err := add(mmark.Provider, "mmark"); err != nil {
+		return nil, err
+	}
+	if err := add(asciidoc.Provider, "asciidoc"); err != nil {
+		return nil, err
+	}
+	if err := add(rst.Provider, "rst"); err != nil {
+		return nil, err
+	}
+	if err := add(pandoc.Provider, "pandoc"); err != nil {
+		return nil, err
+	}
+	if err := add(org.Provider, "org"); err != nil {
+		return nil, err
+	}
+
+	return &converterRegistry{converters: converters}, nil
+}
+
+type ConverterProvider interface {
+	Get(name string) converter.Provider
+}
+
+type converterRegistry struct {
+	// Maps name (md, markdown, blackfriday etc.) to a converter provider.
+	// Note that this is also used for aliasing, so the same converter
+	// may be registered multiple times.
+	// All names are lower case.
+	converters map[string]converter.Provider
+}
+
+func (r *converterRegistry) Get(name string) converter.Provider {
+	return r.converters[strings.ToLower(name)]
+}
+
+func addConverter(m map[string]converter.Provider, c converter.Provider, aliases ...string) {
+	for _, alias := range aliases {
+		m[alias] = c
+	}
+}