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/options.go

@@ -0,0 +1,77 @@
+// 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 option func(m *Markdown)
+
+func HTML(b bool) option {
+	return func(m *Markdown) {
+		m.HTML = b
+	}
+}
+
+func Linkify(b bool) option {
+	return func(m *Markdown) {
+		m.Linkify = b
+	}
+}
+
+func Typographer(b bool) option {
+	return func(m *Markdown) {
+		m.Typographer = b
+	}
+}
+
+func Quotes(stringOrArray interface{}) option {
+	if s, ok := stringOrArray.(string); ok {
+		return func(m *Markdown) {
+			for i, r := range []rune(s) {
+				m.Quotes[i] = string(r)
+			}
+		}
+	}
+	a := stringOrArray.([]string)
+	return func(m *Markdown) {
+		for i, s := range a {
+			m.Quotes[i] = s
+		}
+	}
+}
+
+func MaxNesting(n int) option {
+	return func(m *Markdown) {
+		m.MaxNesting = n
+	}
+}
+
+func XHTMLOutput(b bool) option {
+	return func(m *Markdown) {
+		m.renderOptions.XHTML = b
+	}
+}
+
+func Breaks(b bool) option {
+	return func(m *Markdown) {
+		m.renderOptions.Breaks = b
+	}
+}
+
+func LangPrefix(p string) option {
+	return func(m *Markdown) {
+		m.renderOptions.LangPrefix = p
+	}
+}
+
+func Nofollow(b bool) option {
+	return func(m *Markdown) {
+		m.renderOptions.Nofollow = b
+	}
+}
+
+func Tables(b bool) option {
+	return func(m *Markdown) {
+		m.Tables = b
+	}
+}