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/text.go

@@ -0,0 +1,52 @@
+// 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
+
+var terminatorCharTable = [256]bool{
+	'\n': true,
+	'!':  true,
+	'#':  true,
+	'$':  true,
+	'%':  true,
+	'&':  true,
+	'*':  true,
+	'+':  true,
+	'-':  true,
+	':':  true,
+	'<':  true,
+	'=':  true,
+	'>':  true,
+	'@':  true,
+	'[':  true,
+	'\\': true,
+	']':  true,
+	'^':  true,
+	'_':  true,
+	'`':  true,
+	'{':  true,
+	'}':  true,
+	'~':  true,
+}
+
+func ruleText(s *StateInline, silent bool) bool {
+	pos := s.Pos
+	max := s.PosMax
+	src := s.Src
+
+	for pos < max && !terminatorCharTable[src[pos]] {
+		pos++
+	}
+	if pos == s.Pos {
+		return false
+	}
+
+	if !silent {
+		s.Pending.WriteString(src[s.Pos:pos])
+	}
+
+	s.Pos = pos
+
+	return true
+}