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/strikethrough.go

@@ -0,0 +1,101 @@
+// 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 ruleStrikeThrough(s *StateInline, silent bool) bool {
+	src := s.Src
+	start := s.Pos
+	marker := src[start]
+
+	if silent {
+		return false
+	}
+
+	if src[start] != '~' {
+		return false
+	}
+
+	canOpen, canClose, length := s.scanDelims(start, true)
+	origLength := length
+	ch := string(marker)
+	if length < 2 {
+		return false
+	}
+
+	if length%2 != 0 {
+		s.PushToken(&Text{
+			Content: ch,
+		})
+		length--
+	}
+
+	for i := 0; i < length; i += 2 {
+		s.PushToken(&Text{
+			Content: ch + ch,
+		})
+
+		s.Delimiters = append(s.Delimiters, Delimiter{
+			Marker: marker,
+			Length: -1,
+			Jump:   i,
+			Token:  len(s.Tokens) - 1,
+			Level:  s.Level,
+			End:    -1,
+			Open:   canOpen,
+			Close:  canClose,
+		})
+	}
+
+	s.Pos += origLength
+
+	return true
+
+}
+
+func ruleStrikethroughPostprocess(s *StateInline) {
+	var loneMarkers []int
+	delimiters := s.Delimiters
+	max := len(delimiters)
+
+	for i := 0; i < max; i++ {
+		startDelim := delimiters[i]
+
+		if startDelim.Marker != '~' {
+			continue
+		}
+
+		if startDelim.End == -1 {
+			continue
+		}
+
+		endDelim := delimiters[startDelim.End]
+
+		s.Tokens[startDelim.Token] = &StrikethroughOpen{}
+		s.Tokens[endDelim.Token] = &StrikethroughClose{}
+
+		if text, ok := s.Tokens[endDelim.Token-1].(*Text); ok && text.Content == "~" {
+			loneMarkers = append(loneMarkers, endDelim.Token-1)
+		}
+	}
+
+	for len(loneMarkers) > 0 {
+		i := loneMarkers[len(loneMarkers)-1]
+		loneMarkers = loneMarkers[:len(loneMarkers)-1]
+		j := i + 1
+
+		for j < len(s.Tokens) {
+			if _, ok := s.Tokens[j].(*StrikethroughClose); !ok {
+				break
+			}
+			j++
+		}
+
+		j--
+
+		if i != j {
+			s.Tokens[i], s.Tokens[j] = s.Tokens[j], s.Tokens[i]
+		}
+	}
+}