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/writer.go

@@ -0,0 +1,69 @@
+// 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 (
+	"bufio"
+	"io"
+)
+
+type writer interface {
+	Write([]byte) (int, error)
+	WriteByte(byte) error
+	WriteString(string) (int, error)
+	Flush() error
+}
+
+type monadicWriter struct {
+	writer
+	err error
+}
+
+func newMonadicWriter(w io.Writer) *monadicWriter {
+	if w, ok := w.(writer); ok {
+		return &monadicWriter{writer: w}
+	}
+	return &monadicWriter{writer: bufio.NewWriter(w)}
+}
+
+func (w *monadicWriter) Write(p []byte) (n int, err error) {
+	if w.err != nil {
+		return
+	}
+
+	n, err = w.writer.Write(p)
+	w.err = err
+	return
+}
+
+func (w *monadicWriter) WriteByte(b byte) (err error) {
+	if w.err != nil {
+		return
+	}
+
+	err = w.writer.WriteByte(b)
+	w.err = err
+	return
+}
+
+func (w *monadicWriter) WriteString(s string) (n int, err error) {
+	if w.err != nil {
+		return
+	}
+
+	n, err = w.writer.WriteString(s)
+	w.err = err
+	return
+}
+
+func (w *monadicWriter) Flush() (err error) {
+	if w.err != nil {
+		return
+	}
+
+	err = w.writer.Flush()
+	w.err = err
+	return
+}