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/balance_pairs.go

@@ -0,0 +1,43 @@
+// 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 ruleBalancePairs(s *StateInline) {
+	delimiters := s.Delimiters
+	max := len(delimiters)
+
+	for i := 0; i < max; i++ {
+		lastDelim := delimiters[i]
+
+		if !lastDelim.Close {
+			continue
+		}
+
+		j := i - lastDelim.Jump - 1
+
+		for j >= 0 {
+			currDelim := delimiters[j]
+
+			if currDelim.Open &&
+				currDelim.Marker == lastDelim.Marker &&
+				currDelim.End < 0 &&
+				currDelim.Level == lastDelim.Level {
+				oddMatch := (currDelim.Close || lastDelim.Open) &&
+					currDelim.Length != -1 &&
+					lastDelim.Length != -1 &&
+					(currDelim.Length+lastDelim.Length)%3 == 0
+				if !oddMatch {
+					delimiters[i].Jump = i - j
+					delimiters[i].Open = false
+					delimiters[j].End = i
+					delimiters[j].Jump = 0
+					break
+				}
+			}
+
+			j -= currDelim.Jump + 1
+		}
+	}
+}