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/mdurl/url.go

@@ -0,0 +1,101 @@
+// 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 url provides functions for parsing, decoding and encoding URLs.
+package mdurl
+
+// A URL represents a parsed URL.
+type URL struct {
+	Scheme      string
+	RawScheme   string
+	Slashes     bool
+	Auth        string
+	Host        string
+	Port        string
+	Path        string
+	RawQuery    string
+	HasQuery    bool
+	Fragment    string
+	HasFragment bool
+	IPv6        bool
+}
+
+// String reassembles the URL into a URL string.
+func (u *URL) String() string {
+	size := len(u.Path)
+	if u.Scheme != "" {
+		size += len(u.Scheme) + 1
+	}
+	if u.Slashes {
+		size += 2
+	}
+	if u.Auth != "" {
+		size += len(u.Auth) + 1
+	}
+	if u.Host != "" {
+		size += len(u.Host)
+		if u.IPv6 {
+			size += 2
+		}
+	}
+	if u.Port != "" {
+		size += len(u.Port) + 1
+	}
+	if u.HasQuery {
+		size += len(u.RawQuery) + 1
+	}
+	if u.HasFragment {
+		size += len(u.Fragment) + 1
+	}
+	if size == 0 {
+		return ""
+	}
+
+	buf := make([]byte, size)
+	i := 0
+	if u.Scheme != "" {
+		i += copy(buf, u.Scheme)
+		buf[i] = ':'
+		i++
+	}
+	if u.Slashes {
+		buf[i] = '/'
+		i++
+		buf[i] = '/'
+		i++
+	}
+	if u.Auth != "" {
+		i += copy(buf[i:], u.Auth)
+		buf[i] = '@'
+		i++
+	}
+	if u.Host != "" {
+		if u.IPv6 {
+			buf[i] = '['
+			i++
+			i += copy(buf[i:], u.Host)
+			buf[i] = ']'
+			i++
+		} else {
+			i += copy(buf[i:], u.Host)
+		}
+	}
+	if u.Port != "" {
+		buf[i] = ':'
+		i++
+		i += copy(buf[i:], u.Port)
+	}
+	i += copy(buf[i:], u.Path)
+	if u.HasQuery {
+		buf[i] = '?'
+		i++
+		i += copy(buf[i:], u.RawQuery)
+	}
+	if u.HasFragment {
+		buf[i] = '#'
+		i++
+		i += copy(buf[i:], u.Fragment)
+	}
+	return string(buf)
+}