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/html/html.go

@@ -0,0 +1,211 @@
+// 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 html provides functions for escaping/unescaping HTML text and for parsing HTML entities.
+package html
+
+import (
+	"io"
+	"strconv"
+	"strings"
+	"unicode/utf8"
+)
+
+const BadEntity = string(utf8.RuneError)
+
+var htmlEscapeReplacer = strings.NewReplacer(
+	"&", "&",
+	"<", "&lt;",
+	">", "&gt;",
+	`"`, "&quot;",
+)
+
+func EscapeString(s string) string {
+	return htmlEscapeReplacer.Replace(s)
+}
+
+func WriteEscapedString(w io.Writer, s string) error {
+	_, err := htmlEscapeReplacer.WriteString(w, s)
+	return err
+}
+
+func isValidEntityCode(c int64) bool {
+	switch {
+	case !utf8.ValidRune(rune(c)):
+		return false
+
+	// never used
+	case c >= 0xfdd0 && c <= 0xfdef:
+		return false
+	case c&0xffff == 0xffff || c&0xffff == 0xfffe:
+		return false
+	// control codes
+	case c >= 0x00 && c <= 0x08:
+		return false
+	case c == 0x0b:
+		return false
+	case c >= 0x0e && c <= 0x1f:
+		return false
+	case c >= 0x7f && c <= 0x9f:
+		return false
+	}
+
+	return true
+}
+
+func letter(b byte) bool { return b >= 'a' && b <= 'z' || b >= 'A' && b <= 'Z' }
+
+func digit(b byte) bool { return b >= '0' && b <= '9' }
+
+func alphanum(b byte) bool { return letter(b) || digit(b) }
+
+func hexDigit(b byte) bool {
+	return digit(b) || b >= 'a' && b <= 'f' || b >= 'A' && b <= 'F'
+}
+
+func ParseEntity(s string) (string, int) {
+	st := 0
+	var n int
+
+	for i := 1; i < len(s); i++ {
+		b := s[i]
+
+		switch st {
+		case 0: // initial state
+			switch {
+			case b == '#':
+				st = 1
+			case letter(b):
+				n = 1
+				st = 2
+			default:
+				return "", 0
+			}
+
+		case 1: // &#
+			switch {
+			case b == 'x' || b == 'X':
+				st = 3
+			case digit(b):
+				n = 1
+				st = 4
+			default:
+				return "", 0
+			}
+
+		case 2: // &q
+			switch {
+			case alphanum(b):
+				n++
+				if n > 31 {
+					return "", 0
+				}
+			case b == ';':
+				if e, ok := entities[s[i-n:i]]; ok {
+					return e, i + 1
+				}
+				return "", 0
+			default:
+				return "", 0
+			}
+
+		case 3: // &#x
+			switch {
+			case hexDigit(b):
+				n = 1
+				st = 5
+			default:
+				return "", 0
+			}
+
+		case 4: // &#0
+			switch {
+			case digit(b):
+				n++
+				if n > 8 {
+					return "", 0
+				}
+			case b == ';':
+				c, _ := strconv.ParseInt(s[i-n:i], 10, 32)
+				if !isValidEntityCode(c) {
+					return BadEntity, i + 1
+				}
+				return string(rune(c)), i + 1
+			default:
+				return "", 0
+			}
+
+		case 5: // &#x0
+			switch {
+			case hexDigit(b):
+				n++
+				if n > 8 {
+					return "", 0
+				}
+			case b == ';':
+				c, err := strconv.ParseInt(s[i-n:i], 16, 32)
+				if err != nil {
+					return BadEntity, i + 1
+				}
+				if !isValidEntityCode(c) {
+					return BadEntity, i + 1
+				}
+				return string(rune(c)), i + 1
+			default:
+				return "", 0
+			}
+		}
+	}
+
+	return "", 0
+}
+
+func UnescapeString(s string) string {
+	i := strings.IndexByte(s, '&')
+	if i < 0 {
+		return s
+	}
+
+	anyChanges := false
+	var entityStr string
+	var entityLen int
+	for i < len(s) {
+		if s[i] == '&' {
+			entityStr, entityLen = ParseEntity(s[i:])
+			if entityLen > 0 {
+				anyChanges = true
+				break
+			}
+		}
+		i++
+	}
+
+	if !anyChanges {
+		return s
+	}
+
+	buf := make([]byte, len(s)-entityLen+len(entityStr))
+	copy(buf[:i], s)
+	n := copy(buf[i:], entityStr)
+	j := i + n
+	i += entityLen
+	for i < len(s) {
+		b := s[i]
+		if b == '&' {
+			entityStr, entityLen = ParseEntity(s[i:])
+			if entityLen > 0 {
+				n = copy(buf[j:], entityStr)
+				j += n
+				i += entityLen
+				continue
+			}
+		}
+
+		buf[j] = b
+		j++
+		i++
+	}
+
+	return string(buf[:j])
+}