Custom rendering: strike through support

app/src/main/java/eu/faircode/email/HtmlHelper.java

@@ -1999,6 +1999,9 @@ public class HtmlHelper {
                             case "strong":
                                 ssb.setSpan(new StyleSpan(Typeface.BOLD), start, ssb.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
                                 break;
+                            case "s":
+                                ssb.setSpan(new StrikethroughSpan(), start, ssb.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+                                break;
                             case "u":
                                 ssb.setSpan(new UnderlineSpan(), start, ssb.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
                                 break;