Add support for markdown to HTML conversion (matrix). Closes #663 (#670)

This uses our own gomatrix lib with the SendHTML function which
adds HTML to formatted_body in matrix.
golang-commonmark is used to convert markdown into valid HTML.

vendor/gitlab.com/golang-commonmark/markdown/markdown.go

@@ -0,0 +1,112 @@
+// Copyright 2015 The Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package markdown provides CommonMark-compliant markdown parser and renderer.
+package markdown
+
+import (
+	"bytes"
+	"io"
+)
+
+type Markdown struct {
+	options
+	Block         ParserBlock
+	Inline        ParserInline
+	renderOptions RenderOptions
+}
+
+type RenderOptions struct {
+	LangPrefix string // CSS language class prefix for fenced blocks
+	XHTML      bool   // render as XHTML instead of HTML
+	Breaks     bool   // convert \n in paragraphs into <br>
+	Nofollow   bool   // add rel="nofollow" to the links
+}
+
+type options struct {
+	HTML        bool      // allow raw HTML in the markup
+	Tables      bool      // GFM tables
+	Linkify     bool      // autoconvert URL-like text to links
+	Typographer bool      // enable some typographic replacements
+	Quotes      [4]string // double/single quotes replacement pairs
+	MaxNesting  int       // maximum nesting level
+}
+
+type Environment struct {
+	References map[string]map[string]string
+}
+
+type CoreRule func(*StateCore)
+
+var coreRules []CoreRule
+
+func New(opts ...option) *Markdown {
+	m := &Markdown{
+		options: options{
+			Tables:      true,
+			Linkify:     true,
+			Typographer: true,
+			Quotes:      [4]string{"“", "”", "‘", "’"},
+			MaxNesting:  20,
+		},
+		renderOptions: RenderOptions{LangPrefix: "language-"},
+	}
+	for _, opt := range opts {
+		opt(m)
+	}
+	return m
+}
+
+func (m *Markdown) Parse(src []byte) []Token {
+	if len(src) == 0 {
+		return nil
+	}
+
+	s := &StateCore{
+		Md:  m,
+		Env: &Environment{},
+	}
+	s.Tokens = m.Block.Parse(src, m, s.Env)
+
+	for _, r := range coreRules {
+		r(s)
+	}
+	return s.Tokens
+}
+
+func (m *Markdown) Render(w io.Writer, src []byte) error {
+	if len(src) == 0 {
+		return nil
+	}
+
+	return NewRenderer(w).Render(m.Parse(src), m.renderOptions)
+}
+
+func (m *Markdown) RenderTokens(w io.Writer, tokens []Token) error {
+	if len(tokens) == 0 {
+		return nil
+	}
+
+	return NewRenderer(w).Render(tokens, m.renderOptions)
+}
+
+func (m *Markdown) RenderToString(src []byte) string {
+	if len(src) == 0 {
+		return ""
+	}
+
+	var buf bytes.Buffer
+	NewRenderer(&buf).Render(m.Parse(src), m.renderOptions)
+	return buf.String()
+}
+
+func (m *Markdown) RenderTokensToString(tokens []Token) string {
+	if len(tokens) == 0 {
+		return ""
+	}
+
+	var buf bytes.Buffer
+	NewRenderer(&buf).Render(tokens, m.renderOptions)
+	return buf.String()
+}