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/emphasis.go

@@ -0,0 +1,89 @@
+// 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
+
+type Delimiter struct {
+	Length int
+	Jump   int
+	Token  int
+	Level  int
+	End    int
+	Open   bool
+	Close  bool
+	Marker byte
+}
+
+func ruleEmphasis(s *StateInline, silent bool) bool {
+	src := s.Src
+	start := s.Pos
+	marker := src[start]
+
+	if silent {
+		return false
+	}
+
+	if marker != '_' && marker != '*' {
+		return false
+	}
+
+	canOpen, canClose, length := s.scanDelims(s.Pos, marker == '*')
+	for i := 0; i < length; i++ {
+		s.PushToken(&Text{Content: string(marker)})
+
+		s.Delimiters = append(s.Delimiters, Delimiter{
+			Marker: marker,
+			Length: length,
+			Jump:   i,
+			Token:  len(s.Tokens) - 1,
+			Level:  s.Level,
+			End:    -1,
+			Open:   canOpen,
+			Close:  canClose,
+		})
+	}
+
+	s.Pos += length
+
+	return true
+}
+
+func ruleEmphasisPostprocess(s *StateInline) {
+	delimiters := s.Delimiters
+	max := len(delimiters)
+	for i := max - 1; i >= 0; i-- {
+		startDelim := delimiters[i]
+		if startDelim.Marker != '_' && startDelim.Marker != '*' {
+			continue
+		}
+
+		if startDelim.End == -1 {
+			continue
+		}
+
+		endDelim := delimiters[startDelim.End]
+
+		isStrong := i > 0 &&
+			delimiters[i-1].End == startDelim.End+1 &&
+			delimiters[i-1].Token == startDelim.Token-1 &&
+			delimiters[startDelim.End+1].Token == endDelim.Token+1 &&
+			delimiters[i-1].Marker == startDelim.Marker
+
+		if isStrong {
+			s.Tokens[startDelim.Token] = &StrongOpen{}
+			s.Tokens[endDelim.Token] = &StrongClose{}
+
+			if text, ok := s.Tokens[delimiters[i-1].Token].(*Text); ok {
+				text.Content = ""
+			}
+			if text, ok := s.Tokens[delimiters[startDelim.End+1].Token].(*Text); ok {
+				text.Content = ""
+			}
+			i--
+		} else {
+			s.Tokens[startDelim.Token] = &EmphasisOpen{}
+			s.Tokens[endDelim.Token] = &EmphasisClose{}
+		}
+	}
+}