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/text_collapse.go

@@ -0,0 +1,42 @@
+// 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 ruleTextCollapse(s *StateInline) {
+	level := 0
+	tokens := s.Tokens
+	max := len(tokens)
+
+	curr := 0
+	last := 0
+	for ; curr < max; curr++ {
+		tok := tokens[curr]
+		if tok.Opening() {
+			level++
+		} else if tok.Closing() {
+			level--
+		}
+
+		tok.SetLevel(level)
+
+		if text, ok := tok.(*Text); ok && curr+1 < max {
+			if text2, ok := tokens[curr+1].(*Text); ok {
+				text2.Content = text.Content + text2.Content
+				continue
+			}
+		}
+
+		if curr != last {
+			tokens[last] = tokens[curr]
+		}
+		last++
+	}
+
+	if curr != last {
+		tokens = tokens[:last]
+	}
+
+	s.Tokens = tokens
+}