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/hr.go

@@ -0,0 +1,54 @@
+// 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 ruleHR(s *StateBlock, startLine, endLine int, silent bool) bool {
+	pos := s.BMarks[startLine] + s.TShift[startLine]
+	max := s.EMarks[startLine]
+
+	if pos >= max {
+		return false
+	}
+
+	if s.SCount[startLine]-s.BlkIndent >= 4 {
+		return false
+	}
+
+	src := s.Src
+	marker := src[pos]
+	pos++
+
+	if marker != '*' && marker != '-' && marker != '_' {
+		return false
+	}
+
+	cnt := 1
+	for pos < max {
+		ch := src[pos]
+		pos++
+		if ch != marker && !byteIsSpace(ch) {
+			return false
+		}
+		if ch == marker {
+			cnt++
+		}
+	}
+
+	if cnt < 3 {
+		return false
+	}
+
+	if silent {
+		return true
+	}
+
+	s.Line = startLine + 1
+
+	s.PushToken(&Hr{
+		Map: [2]int{startLine, s.Line},
+	})
+
+	return true
+}