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/image.go

@@ -0,0 +1,131 @@
+// 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 ruleImage(s *StateInline, silent bool) bool {
+	pos := s.Pos
+	max := s.PosMax
+
+	if pos+2 >= max {
+		return false
+	}
+
+	src := s.Src
+	if src[pos] != '!' || src[pos+1] != '[' {
+		return false
+	}
+
+	labelStart := pos + 2
+	labelEnd := parseLinkLabel(s, pos+1, false)
+	if labelEnd < 0 {
+		return false
+	}
+
+	var href, title, label string
+	oldPos := pos
+	pos = labelEnd + 1
+	if pos < max && src[pos] == '(' {
+		pos++
+		for pos < max {
+			b := src[pos]
+			if !byteIsSpace(b) && b != '\n' {
+				break
+			}
+			pos++
+		}
+		if pos >= max {
+			return false
+		}
+
+		start := pos
+		url, _, endpos, ok := parseLinkDestination(src, pos, s.PosMax)
+		if ok {
+			url = normalizeLink(url)
+			if validateLink(url) {
+				href = url
+				pos = endpos
+			}
+		}
+
+		start = pos
+		for pos < max {
+			b := src[pos]
+			if !byteIsSpace(b) && b != '\n' {
+				break
+			}
+			pos++
+		}
+		if pos >= max {
+			return false
+		}
+
+		title, _, endpos, ok = parseLinkTitle(src, pos, s.PosMax)
+		if pos < max && start != pos && ok {
+			pos = endpos
+			for pos < max {
+				b := src[pos]
+				if !byteIsSpace(b) && b != '\n' {
+					break
+				}
+				pos++
+			}
+		}
+
+		if pos >= max || src[pos] != ')' {
+			s.Pos = oldPos
+			return false
+		}
+
+		pos++
+
+	} else {
+		if s.Env.References == nil {
+			return false
+		}
+
+		if pos < max && src[pos] == '[' {
+			start := pos + 1
+			pos = parseLinkLabel(s, pos, false)
+			if pos >= 0 {
+				label = src[start:pos]
+				pos++
+			} else {
+				pos = labelEnd + 1
+			}
+		} else {
+			pos = labelEnd + 1
+		}
+
+		if label == "" {
+			label = src[labelStart:labelEnd]
+		}
+
+		ref, ok := s.Env.References[normalizeReference(label)]
+		if !ok {
+			s.Pos = oldPos
+			return false
+		}
+
+		href = ref["href"]
+		title = ref["title"]
+	}
+
+	if !silent {
+		content := src[labelStart:labelEnd]
+
+		tokens := s.Md.Inline.Parse(content, s.Md, s.Env)
+
+		s.PushToken(&Image{
+			Src:    href,
+			Title:  title,
+			Tokens: tokens,
+		})
+	}
+
+	s.Pos = pos
+	s.PosMax = max
+
+	return true
+}