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/align.go

@@ -0,0 +1,26 @@
+// 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
+
+type Align byte
+
+const (
+	AlignNone = iota
+	AlignLeft
+	AlignCenter
+	AlignRight
+)
+
+func (a Align) String() string {
+	switch a {
+	case AlignLeft:
+		return "left"
+	case AlignCenter:
+		return "center"
+	case AlignRight:
+		return "right"
+	}
+	return ""
+}