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/mdurl/decode.go

@@ -0,0 +1,80 @@
+// 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 mdurl
+
+import (
+	"bytes"
+	"unicode/utf8"
+)
+
+func advance(s string, pos int) (byte, int) {
+	if pos >= len(s) {
+		return 0, len(s) + 1
+	}
+	if s[pos] != '%' {
+		return s[pos], pos + 1
+	}
+	if pos+2 < len(s) &&
+		hexDigit(s[pos+1]) &&
+		hexDigit(s[pos+2]) {
+		return unhex(s[pos+1])<<4 | unhex(s[pos+2]), pos + 3
+	}
+	return '%', pos + 1
+}
+
+// Decode decodes a percent-encoded URL.
+// Invalid percent-encoded sequences are left as is.
+// Invalid UTF-8 sequences are replaced with U+FFFD.
+func Decode(rawurl string) string {
+	var buf bytes.Buffer
+	i := 0
+	const replacement = "\xEF\xBF\xBD"
+outer:
+	for i < len(rawurl) {
+		r, rlen := utf8.DecodeRuneInString(rawurl[i:])
+		if r == '%' && i+2 < len(rawurl) &&
+			hexDigit(rawurl[i+1]) &&
+			hexDigit(rawurl[i+2]) {
+			b := unhex(rawurl[i+1])<<4 | unhex(rawurl[i+2])
+			if b < 0x80 {
+				buf.WriteByte(b)
+				i += 3
+				continue
+			}
+			var n int
+			if b&0xe0 == 0xc0 {
+				n = 1
+			} else if b&0xf0 == 0xe0 {
+				n = 2
+			} else if b&0xf8 == 0xf0 {
+				n = 3
+			}
+			if n == 0 {
+				buf.WriteString(replacement)
+				i += 3
+				continue
+			}
+			rb := make([]byte, n+1)
+			rb[0] = b
+			j := i + 3
+			for k := 0; k < n; k++ {
+				b, j = advance(rawurl, j)
+				if j > len(rawurl) || b&0xc0 != 0x80 {
+					buf.WriteString(replacement)
+					i += 3
+					continue outer
+				}
+				rb[k+1] = b
+			}
+			r, _ := utf8.DecodeRune(rb)
+			buf.WriteRune(r)
+			i = j
+			continue
+		}
+		buf.WriteRune(r)
+		i += rlen
+	}
+	return buf.String()
+}