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/entity.go

@@ -0,0 +1,35 @@
+// 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
+
+import "gitlab.com/golang-commonmark/html"
+
+func ruleEntity(s *StateInline, silent bool) bool {
+	pos := s.Pos
+	src := s.Src
+
+	if src[pos] != '&' {
+		return false
+	}
+
+	max := s.PosMax
+
+	if pos+1 < max {
+		if e, n := html.ParseEntity(src[pos:]); n > 0 {
+			if !silent {
+				s.Pending.WriteString(e)
+			}
+			s.Pos += n
+			return true
+		}
+	}
+
+	if !silent {
+		s.Pending.WriteByte('&')
+	}
+	s.Pos++
+
+	return true
+}