Add workaround to find YouTube channel feeds

YouTube doesn't expose RSS links anymore for new-style URLs.

reader/subscription/finder_test.go

@@ -0,0 +1,21 @@
+// Copyright 2020 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package subscription
+
+import "testing"
+
+func TestFindYoutubeChannelFeed(t *testing.T) {
+	scenarios := map[string]string{
+		"https://www.youtube.com/channel/UC-Qj80avWItNRjkZ41rzHyw": "https://www.youtube.com/feeds/videos.xml?channel_id=UC-Qj80avWItNRjkZ41rzHyw",
+		"http://example.org/feed":                                  "http://example.org/feed",
+	}
+
+	for websiteURL, expectedFeedURL := range scenarios {
+		result := findYoutubeChannelFeed(websiteURL)
+		if result != expectedFeedURL {
+			t.Errorf(`Unexpected Feed, got %s, instead of %s`, result, expectedFeedURL)
+		}
+	}
+}