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/linkify/util.go

@@ -0,0 +1,20 @@
+// 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 linkify
+
+func digit(b byte) bool {
+	return b >= '0' && b <= '9'
+}
+
+func hexDigit(b byte) bool {
+	return digit(b) || b >= 'a' && b <= 'f' || b >= 'A' && b <= 'F'
+}
+
+func byteToLower(b byte) byte {
+	if b >= 'A' && b <= 'Z' {
+		return b - 'A' + 'a'
+	}
+	return b
+}