add_mailto_subject: New rewrite function

Dinosaur Comics (qwantz.com) likes to hide jokes in mailto: links, but
miniflux's sanitizer strips those out.

reader/rewrite/rewrite_functions.go

@@ -7,6 +7,7 @@ package rewrite // import "miniflux.app/reader/rewrite"
 import (
 	"fmt"
 	"html"
+	"net/url"
 	"regexp"
 	"strings"
 
@@ -43,6 +44,38 @@ func addImageTitle(entryURL, entryContent string) string {
 	return entryContent
 }
 
+func addMailtoSubject(entryURL, entryContent string) string {
+	doc, err := goquery.NewDocumentFromReader(strings.NewReader(entryContent))
+	if err != nil {
+		return entryContent
+	}
+
+	matches := doc.Find(`a[href^="mailto:"]`)
+
+	if matches.Length() > 0 {
+		matches.Each(func(i int, a *goquery.Selection) {
+			hrefAttr, _ := a.Attr("href")
+
+			mailto, err := url.Parse(hrefAttr)
+			if err != nil {
+				return
+			}
+
+			subject := mailto.Query().Get("subject")
+			if subject == "" {
+				return
+			}
+
+			a.AppendHtml(" [" + html.EscapeString(subject) + "]")
+		})
+
+		output, _ := doc.Find("body").First().Html()
+		return output
+	}
+
+	return entryContent
+}
+
 func addDynamicImage(entryURL, entryContent string) string {
 	doc, err := goquery.NewDocumentFromReader(strings.NewReader(entryContent))
 	if err != nil {