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/encode.go

@@ -0,0 +1,53 @@
+// 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"
+	"strings"
+	"unicode/utf8"
+)
+
+// Encode percent-encodes rawurl, avoiding double encoding.
+// It doesn't touch:
+// - alphanumeric characters ([0-9a-zA-Z]);
+// - percent-encoded characters (%[0-9a-fA-F]{2});
+// - excluded characters ([;/?:@&=+$,-_.!~*'()#]).
+// Invalid UTF-8 sequences are replaced with U+FFFD.
+func Encode(rawurl string) string {
+	const hexdigit = "0123456789ABCDEF"
+	var buf bytes.Buffer
+	i := 0
+	for i < len(rawurl) {
+		r, rlen := utf8.DecodeRuneInString(rawurl[i:])
+		if r >= 0x80 {
+			for j, n := i, i+rlen; j < n; j++ {
+				b := rawurl[j]
+				buf.WriteByte('%')
+				buf.WriteByte(hexdigit[(b>>4)&0xf])
+				buf.WriteByte(hexdigit[b&0xf])
+			}
+		} else if r == '%' {
+			if i+2 < len(rawurl) &&
+				hexDigit(rawurl[i+1]) &&
+				hexDigit(rawurl[i+2]) {
+				buf.WriteByte('%')
+				buf.WriteByte(byteToUpper(rawurl[i+1]))
+				buf.WriteByte(byteToUpper(rawurl[i+2]))
+				i += 2
+			} else {
+				buf.WriteString("%25")
+			}
+		} else if strings.IndexByte("!#$&'()*+,-./0123456789:;=?@ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz~", byte(r)) == -1 {
+			buf.WriteByte('%')
+			buf.WriteByte(hexdigit[(r>>4)&0xf])
+			buf.WriteByte(hexdigit[r&0xf])
+		} else {
+			buf.WriteByte(byte(r))
+		}
+		i += rlen
+	}
+	return buf.String()
+}