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/autolink.go

@@ -0,0 +1,70 @@
+// 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
+
+import (
+	"regexp"
+	"strings"
+)
+
+var (
+	rAutolink = regexp.MustCompile(`^<([a-zA-Z][a-zA-Z0-9+.\-]{1,31}):([^<>\x00-\x20]*)>`)
+	rEmail    = regexp.MustCompile(`^<([a-zA-Z0-9.!#$%&'*+/=?^_{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*)>`)
+)
+
+func ruleAutolink(s *StateInline, silent bool) bool {
+	pos := s.Pos
+	src := s.Src
+
+	if src[pos] != '<' {
+		return false
+	}
+
+	tail := src[pos:]
+
+	if strings.IndexByte(tail, '>') < 0 {
+		return false
+	}
+
+	link := rAutolink.FindString(tail)
+	if link != "" {
+		link = link[1 : len(link)-1]
+		href := normalizeLink(link)
+		if !validateLink(href) {
+			return false
+		}
+
+		if !silent {
+			s.PushOpeningToken(&LinkOpen{Href: href})
+			s.PushToken(&Text{Content: normalizeLinkText(link)})
+			s.PushClosingToken(&LinkClose{})
+		}
+
+		s.Pos += len(link) + 2
+
+		return true
+	}
+
+	email := rEmail.FindString(tail)
+	if email != "" {
+		email = email[1 : len(email)-1]
+		href := normalizeLink("mailto:" + email)
+		if !validateLink(href) {
+			return false
+		}
+
+		if !silent {
+			s.PushOpeningToken(&LinkOpen{Href: href})
+			s.PushToken(&Text{Content: normalizeLinkText(email)})
+			s.PushClosingToken(&LinkClose{})
+		}
+
+		s.Pos += len(email) + 2
+
+		return true
+	}
+
+	return false
+}