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/html_inline.go

@@ -0,0 +1,57 @@
+// 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"
+
+var (
+	attrName     = `[a-zA-Z_:][a-zA-Z0-9:._-]*`
+	unquoted     = "[^\"'=<>`\\x00-\\x20]+"
+	singleQuoted = `'[^']*'`
+	doubleQuoted = `"[^"]*"`
+	attrValue    = `(?:` + unquoted + `|` + singleQuoted + `|` + doubleQuoted + `)`
+	attribute    = `(?:\s+` + attrName + `(?:\s*=\s*` + attrValue + `)?)`
+	openTag      = `<[A-Za-z][A-Za-z0-9-]*` + attribute + `*\s*/?>`
+	closeTag     = `</[A-Za-z][A-Za-z0-9-]*\s*>`
+	comment      = `<!---->|<!--(?:-?[^>-])(?:-?[^-])*-->`
+	processing   = `<[?].*?[?]>`
+	declaration  = `<![A-Z]+\s+[^>]*>`
+	cdata        = `<!\[CDATA\[[\s\S]*?\]\]>`
+	rHTMLTag     = regexp.MustCompile(`^(?:` + openTag + `|` + closeTag + `|` + comment +
+		`|` + processing + `|` + declaration + `|` + cdata + `)`)
+)
+
+func htmlSecond(b byte) bool {
+	return b == '!' || b == '/' || b == '?' || isLetter(b)
+}
+
+func ruleHTMLInline(s *StateInline, silent bool) bool {
+	if !s.Md.HTML {
+		return false
+	}
+
+	pos := s.Pos
+	src := s.Src
+	if pos+2 >= s.PosMax || src[pos] != '<' {
+		return false
+	}
+
+	if !htmlSecond(src[pos+1]) {
+		return false
+	}
+
+	match := rHTMLTag.FindString(src[pos:])
+	if match == "" {
+		return false
+	}
+
+	if !silent {
+		s.PushToken(&HTMLInline{Content: match})
+	}
+
+	s.Pos += len(match)
+
+	return true
+}