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/backticks.go

@@ -0,0 +1,60 @@
+// 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 ruleBackticks(s *StateInline, silent bool) bool {
+	pos := s.Pos
+	src := s.Src
+
+	if src[pos] != '`' {
+		return false
+	}
+
+	start := pos
+	pos++
+	max := s.PosMax
+
+	for pos < max && src[pos] == '`' {
+		pos++
+	}
+
+	marker := src[start:pos]
+
+	matchStart := pos
+	matchEnd := pos
+	for {
+		matchStart = strings.IndexByte(src[matchEnd:], '`')
+		if matchStart == -1 {
+			break
+		}
+		matchStart += matchEnd
+
+		matchEnd = matchStart + 1
+
+		for matchEnd < max && src[matchEnd] == '`' {
+			matchEnd++
+		}
+
+		if matchEnd-matchStart == len(marker) {
+			if !silent {
+				s.PushToken(&CodeInline{
+					Content: normalizeInlineCode(src[pos:matchStart]),
+				})
+			}
+			s.Pos = matchEnd
+			return true
+		}
+	}
+
+	if !silent {
+		s.Pending.WriteString(marker)
+	}
+
+	s.Pos += len(marker)
+
+	return true
+}