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/inline.go

@@ -0,0 +1,13 @@
+// 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
+
+func ruleInline(s *StateCore) {
+	for _, tok := range s.Tokens {
+		if tok, ok := tok.(*Inline); ok {
+			tok.Children = s.Md.Inline.Parse(tok.Content, s.Md, s.Env)
+		}
+	}
+}