fix: Include type for OPML subscriptions

As per [OPML 2.0 specification]:

> Each sub-element of the body of the OPML document is a node of type rss or an outline element that contains nodes of type rss.

> Required attributes: type, text, xmlUrl.

[OPML 2.0 specification]: http://opml.org/spec2.opml#subscriptionLists

internal/reader/opml/opml.go

@@ -34,6 +34,23 @@ type opmlOutline struct {
 	Outlines opmlOutlineCollection `xml:"outline,omitempty"`
 }
 
+func (outline opmlOutline) MarshalXML() ([]byte, error) {
+	type opmlOutlineXml opmlOutline
+
+	outlineType := ""
+	if IsSubscription(outline) {
+		outlineType = "rss"
+	}
+
+	return xml.Marshal(struct {
+		opmlOutlineXml
+		Type string `xml:"type,attr,omitempty"`
+	}{
+		opmlOutlineXml: opmlOutlineXml(outline),
+		Type: outlineType,
+	})
+}
+
 func (o *opmlOutline) IsSubscription() bool {
 	return strings.TrimSpace(o.FeedURL) != ""
 }