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/code.go

@@ -0,0 +1,37 @@
+// 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 ruleCode(s *StateBlock, startLine, endLine int, _ bool) bool {
+	if s.SCount[startLine]-s.BlkIndent < 4 {
+		return false
+	}
+
+	nextLine := startLine + 1
+	last := nextLine
+
+	for nextLine < endLine {
+		if s.IsLineEmpty(nextLine) {
+			nextLine++
+			continue
+		}
+
+		if s.SCount[nextLine]-s.BlkIndent >= 4 {
+			nextLine++
+			last = nextLine
+			continue
+		}
+
+		break
+	}
+
+	s.Line = last
+	s.PushToken(&CodeBlock{
+		Content: s.Lines(startLine, last, 4+s.BlkIndent, true),
+		Map:     [2]int{startLine, s.Line},
+	})
+
+	return true
+}