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/escape.go

@@ -0,0 +1,61 @@
+// 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 "strings"
+
+func escaped(b byte) bool {
+	return strings.IndexByte("\\!\"#$%&'()*+,./:;<=>?@[]^_`{|}~-", b) != -1
+}
+
+func ruleEscape(s *StateInline, silent bool) bool {
+	pos := s.Pos
+	src := s.Src
+
+	if src[pos] != '\\' {
+		return false
+	}
+
+	pos++
+	max := s.PosMax
+
+	if pos < max {
+		b := src[pos]
+
+		if b < 0x7f && escaped(b) {
+			if !silent {
+				s.Pending.WriteByte(b)
+			}
+			s.Pos += 2
+			return true
+		}
+
+		if b == '\n' {
+			if !silent {
+				s.PushToken(&Hardbreak{})
+			}
+
+			pos++
+
+			for pos < max {
+				b := src[pos]
+				if !byteIsSpace(b) {
+					break
+				}
+				pos++
+			}
+
+			s.Pos = pos
+			return true
+		}
+	}
+
+	if !silent {
+		s.Pending.WriteByte('\\')
+	}
+	s.Pos++
+
+	return true
+}