Add addDynamicIframe rewrite function.

Add unit tests for `add_dynamic_iframe` rewrite.

internal/reader/rewrite/rewrite_functions.go

@@ -167,6 +167,43 @@ func addDynamicImage(entryURL, entryContent string) string {
 	return entryContent
 }
 
+func addDynamicIframe(entryURL, entryContent string) string {
+	doc, err := goquery.NewDocumentFromReader(strings.NewReader(entryContent))
+	if err != nil {
+		return entryContent
+	}
+
+	// Ordered most preferred to least preferred.
+	candidateAttrs := []string{
+		"data-src",
+		"data-original",
+		"data-orig",
+		"data-url",
+		"data-lazy-src",
+	}
+
+	changed := false
+
+	doc.Find("iframe").Each(func(i int, iframe *goquery.Selection) {
+		for _, candidateAttr := range candidateAttrs {
+			if srcAttr, found := iframe.Attr(candidateAttr); found {
+				changed = true
+
+				iframe.SetAttr("src", srcAttr)
+
+				break
+			}
+		}
+	})
+
+	if changed {
+		output, _ := doc.Find("body").First().Html()
+		return output
+	}
+
+	return entryContent
+}
+
 func fixMediumImages(entryURL, entryContent string) string {
 	doc, err := goquery.NewDocumentFromReader(strings.NewReader(entryContent))
 	if err != nil {